One of the rudest things (musically speaking) that happened to me was when I was singing a song (accompanying myself on guitar) at a pace I like for that particular song: a young woman opposite had obviously only ever known the disco version and started pounding fiendishly with her foot on a wooden floor to try to make me go about twice as fast. I know what I should have done, but in those days was fairly new to the session scene, so didn't do much other than give her the "daggers" stare, which made no difference!
As for people who "jump in": it wouldn't work at one of the sessions I go to. It is done in strict rotation round the room for those there at the start: anyone who arrives late, gets their name added to the bottom of the list by the session chairperson, so doesn't matter where they are sitting, they will still be last! Anyone trying to go out of turn will be deprived of the right to take a turn for the rest of the evening.
As for G# (or even A flat), I'm reminded of a former moothie player (sadly no longer with us) who played in many keys, but would shake his head and say, "Och, they're in one of they shitey keys again".