The thing that I find a problem is young children on the dance floor. The sort that toddle through the dancers to find Mummy, then are allowed to wander off again.
It amazes me that parents don't see it as a danger, and are blind to hints and mild requests. Most dancers are not looking at that level, so could trip.
I've not seen any accident caused like that (says she, tempting fate).
One thing I'm wary of, since someone in the bottom set fell and broke a bone, is the tendency for sets to spread down the room and squash the end set. Espescially in dances like Barley Reel with an 'Arch at the bottom, cast and lead up' figure.