-I did this one once, and cringed about for months: insisting that someone sing "her" (regular) song when she has said she doesn't feel like it tonight.
-When you make a social error in a session and no-one tells you about it but everyone sits there visibly disapproving in some private but obviously unanimous way.
-When someone doesn't make any effort to find out how this session works before they start.
-When a session in general seems to think that how to behave in a session situation is a human instinct and anyone who hasn't worked it out within a fourteenth of a second after walking in must be suffering from a congenital deformity and is probably psychopathic.

That is why this thread is extremely useful - already there have been gripes about
1) B.O. but also perfumes (a surprising number of people are highly allergic)
2) talkers but also anti-talking police
3) more generally lack of consideration but also intolerance

How to behave will always be a compromise between the extremes of "everything's OK and anyone who tries to regulate my behaviour is a fascist" and the equally intolerable "anything which requires the least amount of patience or forgiveness is unacceptable". Obviously good manners is finding the local agreement on where to draw the line (and how braod the line is).