The sign on the elevator in an apartment building I just visited has a list by the floor buttons. Along with the usual suspects, you're not supposed to flush kitty litter or cigarette butts down the toilet.
Since I'm not a smoker, I don't know if it would have ever occurred to me to flush a butt of the cigarette variety down a toilet. But I've been a cat owner all of my life and seriously, it never once occurred to me to flush litter down the toilet.
My kids, however, did flush my keys down the toilet once. So there's that.