It's accordionists with no sense of rhythm but who insist of making random stabs at the LH buttons that are more of a problem.

I've heard more than one story of someone with a very large PA joining a session and playing it with about as much subtlety as a herd of stampeding elephants and ruining a session. So far, I've been lucky not to encounter such a person, though I have come across one or two box players who can't leave the LH alone, even though it would have been better to have done so.

There was the guy at Saltburn this year who joined a session with a guitar and a Moothy on a rack and proceeded to play the Moothy by alternately sucking and blowing (literally - suck, blow, suck, blow...) with no sense of either tune or rhythm. I was told later by someone else in the session that he nearly ruined a singaround later in the day. Now there's an achievement for you.