Bullets.
As a Nerfer, I have begun to dislike this word mainly because people often refer to Nerf darts as "bullets", when they're not. Even worse, I have seen some people refer to other types of Nerf ammunition, namely Vortex discs, as bullets when they're no where close to them. It don't help that my family deliberately calls them bullets just to annoy me, I mean I know they're joking, but it gets old after a while.
Now to be fair, I can easily see why people would call them bullets, especially when they're new to Nerf. If it's just once that's not bad, but when it's constantly misused, it gets kind of annoying. They're called darts, not bullets!