This goes back a few years but still...
While I was working at the pet shop, my mother walked in carrying her bunny in her arms. I could see that something wasn't right. The bunny had the famous lil ball (you know the one with the holes around and bell inside) stuck to her mouth. When I checked her out, I saw that her teeth were badly caught in one of the holes and the ball was almost cutting into her gums. We had to restrain her and cut the ball off with pliers.
I never let the bunnies play with their toys unsurpervised and neither did my mom, but the bunny just wanted to toss the ball so my mother didn't think anything of it.
So anywho, I thought I'd mention it because it is such a common toy. I suggest you get one with larger holes.