My top favourite most worsted bad manners things are:

Guitarists playing random lead over the top of a song just because they can identify(ish) the chords & melody and have a few licks that they think will fit.

People who come late to singarounds/open mics and then expect to "catch up" by doing extra sets late in the evening. Those of us that get there on time & work on getting the atmosphere going REALLY resent the "not yet" brigade. Ditto those that have to be begged to perform.

Percussionists that impose their own rhythm onto your song. Especially those that impose the same rhythm over everything all night.

People who haven't bothered to prepare, and think its OK to sing something from paperwork for the very first time - along with halts & restarts & tailings off when they find they don'd know the songs as well as they had thought they did.

Actually - most things are bearable really - but it does us good to have a bit of a soundoff thread every now & then to get it off our chests.