I hate people who use flash, or even more commonly and annoyingly the flashlight on their phone, in nocturnal areas of zoos, which have large signs saying not to do so, I might add. I suppose some of the animals have gotten used to it, such as when a guy shined the flashlight of his phone on an aye aye at the Omaha zoo and it just stared him down, but most can't take the sudden flashes and freak out, such as when another guy at the Omaha zoo managed to make every species on exhibit ahead of me hide with his stupid phone flashlight to amuse his kids.
This also applies to a general ignorance of signs, which spans outside of zoos. Even in a park near me, where there is a sign saying not to feed the waterfowl, stating why, AND that it's against the law, local residents and visitors have made it a hobby to feed the waterfowl bread, which has had noticeable affects to the local coot population such as severe overcrowding.