I've been keeping balls for about 6 years now and in all that time I can recall being bitten on 3 occasions.

Once was by a young normal ball that was very timid and he nipped my finger as I moved some furniture around his cage.

The other two times was by the same spider ball, and both were feeding response bites where he latched on. He did this once when he was a baby and then again in June 2013.