(1)Enneagram tests don't work.
A common thinking among fans of the enneagram is that tests don't work. The underlying premise is that because some people disagree with their test results and/or their typing of others is incongruent with what that person's test results say, tests therefore must not work. This involves both a bias (e.g. "either my judgment is wrong or the tests are wrong, therefore the tests must be wrong") and a kind of either/or thinking.
It's true that some tests don't work as well as others. In general, the free tests you get on sites like similarminds or quizilla are less likely to be accurate than tests like the RHETI. The best tests will be ones that have been scientifically validated, and where the person writing the test knows both a lot about the enneagram and a lot about test construction. If the test is well written, for most people, your type will either be the highest score, or in the top three.
Some people object that tests aren't valid because people don't know themselves very well, and/or the questions are too easy to guess. While some of the questions may be obvious, they will be most obvious to the enneagram veteran--not to the newcomer who is just starting to figure out their type. Also, as long as people are taking the test for personal pleasure and/or insight, there's very little impetus to fudge answers. If you're taking a psychological test as a pre-employment screening or part of a custody battle, then honesty is a concern. However, for the average person taking a ten dollar test as part of self-exploration, coming across a certain way will be less of a concern. Similarly, although everyone has aspects of themselves unknown to them, a good test is constructed so that the average adult of average intelligence can adequately participate and have relatively reliable results.
If you take an enneagram test, and think that your highest score is not indicative of your personality, certainly, you shouldn't accept that as your type. I think it's a good idea to always read the descriptions of your top three and see which fits the most. At the same time, if, say, you're convinced you're a nine and nine is one of your lowest scores, then you might want to examine the types and yourself more in-depth to make sure your self-typing is correct. Especially be careful of accepting others typing of you if the test data doesn't bear it out. Remember that the test questions correspond to how people of a given type, statistically, most commonly respond, so at bare minimum, your type should be in the top 35-50% of your results.
(2)Your enneagram type is most easily detectable when you're young.
Actually, when you're young--under 21 or 22--your personality is still considerably fluid. Developmental theory tells us that kids are settling into adulthood at later ages than previously, probably because of the average human lifespan being longer due to scientific breakthroughs. Furthermore, certain types--2,4 and 5 especially--correspond to the sorts of issues that teenagers typically tangle with. Someone experiencing an especially turbulent adolescence might identify themselves as a 4 when they're sixteen, only to "outgrow" a lot of the typical four feelings and identify themselves as a 1 or a 3 at twenty-two. For this reason, I think the ideal age to determine type is between 23-26. That's not to say that you can't know your type when young, but realize that (especially if you're high school aged or younger) your personality may still be forming.
(3)Sixes frequently mistype themselves as four, so if you think you're a four, look at six.
While this can happen sometimes, it's more common for other people to mistype fours as sixes than it is for sixes to mistype themselves as fours. Some sixes might mistype themselves as fours initially, since both sixes and fours experience a lot of emotional turbulence, but sixes typically find their type rather quickly once they've read the enneagram literature. Contrary to popular belief, sixes actually don't want to see themselves in the four identification. Sixes place a high premium on loyalty, and want to see themselves as trustworthy people who can be relied on. They also tend to dislike what they consider to be "pretension." Considering this, it's hard to see how they could identify with the four descriptions of self-absorption, lack of productivity and aesthetic elitism. Furthermore, sixes frequently over-identify with their anxiety, which makes the match an easy one to make.
(4)Everyone wants to be a four or a five, because those are traits society idealizes.
This is something of a Riso & Hudson-ism which a lot of enneagram fans have adopted. Actually, it's not so much that everyone wants to be a four or a five, as it is that Riso & Hudson, as many contributions as they've made, import too many cultural and subcultural stereotypes into their type descriptions. As much as I've learned from their books, and as much as I loved Personality Types when I read it, they seriously muddied the waters by over-emphasizing the artistic inclinations of four and intellectualism of five, while relegating types 3 and 6 to various generic corporate roles. Your average struggling artist or aspiring philosopher is hardly going to find much to relate to in the descriptions of types 3 or 6. However, once they read the more psychologically oriented enneagram theorists (e.g. Naranjo and Maitri), they'll see how the underlying issues of those types might not be relevant to them and correct the typing.
(5)You can tell someone's enneagram type by their writing style.
The only thing you can tell by someone's writing style is, well, their writing style. If you're talking about great literary figures, you need to remember that much of this didn't come out of thin air. Coleridge was part of a prominant literary movement that informed his poetry. Kierkegaard was directly engaging with figures at the time, and his unsystematic style of writing was a direct commentary on the systematic philosophy that he saw as both philosophically problematic and a threat to faith. Biographies can be more helpful in this regard, but going on writing style alone--especially if you don't know the historical context in which the works were written--can be incredibly misleading. Even worse is trying to type people you meet online through their writing style. Blogging is hardly indicative of one's true self. People adopt personas that at best don't give the whole picture and at worst, may be completely fabricated. Furthermore, the complete lack of spontaneity or observable non-verbal communication makes getting a read on someone online exceedingly difficult. That's not to say you can't speculate, or that typing online is an impossibility, but you need a lot more than just writing style to go on.
(6)You can tell someone's type by how they act in an argument.
This makes a major assumption: that people act the same way in every conflict situation. While it's true that there everyone has certain conflict styles that are individual to them, many conflict situations are an interplay between how their style matches with the other person's style. Each person plays off the other in a way that either pushes buttons or smooths over the troubles, and different types of conflict situations can elicit stronger or more subdued reactions based on how they feel about the other person, how compatible that person's conflict style is with theirs and how emotionally charged the source of the conflict is. Furthermore, social norms about the type of reaction may temper it as well--someone may act differently in an online forum than they would at work, or at work than they would with a family member or significant other. The only way to gain insight to someone's personality through conflict is to look at how they are over time, in a broad spectrum of situations, with a variety of people.
(7)You can tell someone's enneatype by their picture.
Although phototyping can be fun in a non-serious sort of way, becoming invested in your typing of someone based on their photo is a recipe for error. I've seen people type based on online photographs that clearly made liberal use of photoshop. Even if you could base your typing on things like features, mannerisms or how someone carries themselves, what you're seeing is probably quite different from how they would look if you saw them on the subway or sitting in a coffeehouse. Additionally, features such as eyebrows, mouth, build, etcetera tend to run along genetic lines, and whereas genetics does seem to play a role in the formation of personality, there's a lot more at work, especially in motivation-based theories like the enneagram. By that rationale, every woman on my dad's side of the family should be the same enneatype, since our physical features are strikingly similar, yet we have strikingly different motivations.
(8)It's an art, not a science
This either/or thinking has never made much sense to me. Why do art and science have to be mutually exclusive? The underlying belief seems to be that art is subjective and leaves a lot of room for both creativity and interpretation, whereas science employs raw, factual data and is objective. However, there is still some objectivity in the arts, and a specific structure and methodology in what makes something good. It's how we can say that the fifteen year old goth kid writing love poems at Denny's is a bad writer, whereas Goethe was a highly talented and influential literary figure. Similarly, good work in the sciences requires creativity, the ability to synthesize past research, and finding new interpretations for old data in order to formulate theories and develop experiments. In most any field, the arts and sciences are interdependent, not separate. Looking at scientific data for the enneagram can aid in our own self-evaluations as well as intellectual inquiries of the system. To try to make it either all art or all science is to cut out a huge source of knowledge.
(9)If you're x Myers-Briggs type, your enneatype must be y.
The important thing to consider is that these are merely correlations. Just because INFJ is commonly associated with types one and four doesn't mean that being INFJ makes you a one or a four, or that if you're an ENFP you can't possibly be a one. It just means that there's a statistical significance between the type on one system and the type on another. However, statistical significance can be relatively small (as little as 20% or less) so while it is more common for certain people to have a given Myers-Briggs type and a given enneatype, there will be a fair amount of people that have a Myers-Briggs type that doesn't fit the correlations. Furthermore, there is considerable disagreement among enneagram theorists on which Myers-Briggs types correspond to which enneatypes. If enneatypes and Myers-Briggs types were identical, we wouldn't need different constructs to explain personality.
Similarly, you can't determine type by extroversion and introversion. While some people are very clearly introverted and extroverted, others may vary based on their life situation and how they adapt to choices made in terms of work, relationships, etcetera. Furthermore, roles often confuse determining whether we're dealing with an extrovert or an introvert. For example, most teachers look extroverted because they're up in front of a class, engaging with students, and talking a lot. If you were to see them working in the library, or at home with their family, you might get a totally different impression, however. A four can be extroverted and a two can be introverted, albeit less commonly. Remember that the enneagram is a system of motivation, not traits or information processing, and there are no hard and fast rules about introversion and extroversion with motivation.
(10)If you can't figure out your type, you must be a 3, 6 or 9.
Sometimes a 3, 6 or 9 can be more susceptible to type confusion. The reason a 3 might have trouble identifying their type is because they confuse their projected image for their inner self. A six might have trouble because they overthink and second-guess. A nine might have trouble because they identify with all of the types. However, anyone can have trouble identifying their types. I've known threes to immediately identify their types, and I've seen sixes that quickly discern they're sixes with minimal confusion. I've also seen fours that are in a constant state of questioning their type, due to the four tendency towards identity problems. In general, any type can be confused about their type, or type themselves incorrectly. There are a number of reasons for this. The biggest reason is not having delved deeply enough into the literature and got a good picture of the issues, motivations and defense mechanisms of the types. Another reason is when there's a disjunct between their typing and how they're typed by others, causing them to re-examine whether they've accurately typed themselves. Yet another reason is if they get too literal with the literature, expecting it to encompass everything about themselves, so if they find they don't relate to certain points, start looking to other types trying to find a perfect "fit." If one adequately studies the literature, gets helpful feedback from those closest to them, and realizes how there is always some variability among even people of the same type, then it becomes much easier to identify one's type.