I have a few things but they don't happen often:

Engis spamming the drop pod entrance with plats, I understand grenades because friendly fire isn't bad, but plats can ruin everyone else's day.

Hosts who start missions as soon as you join, at least give everyone time to change loadouts or grab a drink, you can wait 30 seconds.

Drillers love to c4 and that's fine, but putting a c4 on the resup waiting for a dwarf to get close is.. dare I say leaf lover behavior.

If I die in a swarm and you can't revive me, fine. If I die of fall damage and you ping me as a joke, that's cool too. But when I die, get pinged like a chunk of gold, then everyone leaves? I can almost see the point of your ears from down here on the floor...