The one that drives me truly batshit is a negative reaction to anyone who says that they want to lose weight, takes steps to lose weight, or actually loses weight. I've seen this behavior in lots of fat activists, and it seems truly hypocritical to me. They have effectively inverted the dominant paradigm and shifted to size acceptance only for fat people. They don't see being less-fat as an acceptable choice, and they think that anyone who makes that choice is either a traitor to the cause or an idiot who doesn't know that fat is wonderful. These activists are fighting against society's one-true-wayisms with a different flavor of one-true-wayism.
A sub-behavior is questioning the motivations of anyone who chooses to lose weight. As I see it, there are plenty of valid reasons to want to lose weight, and any reason that someone chooses should be perfectly acceptable-- only that person gets to make decisions about their body. I've seen many fat activists assert, however, that the real reason is that they're caving to social pressure or they don't know better.
I'm also annoyed when people assert that weight is not a choice, and that there is nothing that one can do to change their weight. I count among my friends several examples of people who have made choices that either caused them to lose or gain a great deal of weight. Anyone who asserts that weight is completely predetermined, and that nobody can make choices about their weight, is an idiot.
I've covered this one before, but it still irks me when people assert that there are no possible negative health consequences of being overweight. This assertion clearly flies in the face of medical evidence, logic, and the laws of physics, and yet some people continue to repeat it. However, I've given up on caring about people who say, "Well I'm fat, and I don't have health problems because of it, so obviously being fat can't cause health problems." They're just people who are bad at both math and logic.
The one that I've encountered a few times recently is preaching fat acceptance in inappropriate venues. I saw two examples of this in the last week-- someone who wanted to use her (not-fat-activist-related) job as a platform for preaching fat activism to people she did business with. Another was someone who was bothered by a weight loss support group on a discussion forum, and wanted to drop in and share fat activist information. Both of these seemed completely out-of-line to me. I'd be irked if my UPS delivery guy used his position try to sway me about dieting, and it would be inappropriate for me to go to a synagogue to tell people that Jesus Christ is their personal savior. Activism has its place, and there are also places where it is wildly inappropriate.
There's a sub-behavior of this one too. "Well, obviously if they knew about size acceptance, they would let go of their ridiculous desire to lose weight. Clearly they've never heard the gospel, so I must share it with them." Umm, no. It's entirely possible that they've heard your arguments, read your literature, and don't agree with all of your conclusions.
OK, I've ranted. D, I hope I haven't pissed you off too much this time-- I tried to focus on specific behaviors, rather than screaming about the whole FA community.