Chairs with arms that get in the way of your elbows - whether trendy or not (the chairs. The trendiness of instrumentalists' elbows is a separate issue).

My current session (the Golden Lion, Lancaster, Sunday evening) is in a very small room, containing a large sofa and 3 soft armchairs. We tend to end up with people perched on the arms of the sofa, on the window-ledge, on stools, while the supposed seating remains empty. Which is silly but, as pointed out above, isn't going to bother the pub much because we're not a major source of income.

And then, offtopically, the presence of empty comfortable chairs encourages a secondary issue - spectators. "Ooh, Folk Music, how nice"; being a v. small room, they then feel obliged to interact with us ...