Neoprene girths, specifically.
I had a brief stint where someone talked me into using a neoprene girth on my little mare (she has no withers to speak of, and even outfitted with a breast collar, her saddle can get to swaying from time to time), and after awhile, it actually started ripping out her fur and now she has white marks along her girth line. Needless to say, that girth was replaced with a felt one right quick.
Then there's my gelding, who I also used a neoprene girth on around the same time. But only for one or two rides... I think he might be allergic to the neoprene, since he gets a bunch of bumps, kinda like hives, and really touchy wherever the neoprene lays against him.
Since I like to keep things like girths and pads so that they can potentially be switched between horses, I've banished neoprene from my tack room.
Has anyone else had problems with neoprene-anything? Other than sticking to the horse a little better, what purpose does it even serve? It doesn't seem to breath at all, and the only upside I've found is that it's easy to clean.