I've been bitten once by my ball. I believe it was due to my conditioning of her.
I've had my ball for over a year now. 99% of the time I only handle her on feeding days.
On feeding day, I take her out and let her move around on my hands and arms. I do this for 30 Mins or so. Then I put her in an empty tank to feed her and give her a FK rat that is still warm.
The day I got bit is when I took my animal out on a non feeding day. I took her out and handled her on a nonfeeding day just to clean the cage. I put her back in her enclosure.
I was looking everything over when I noticed something needed to still be moved. When I put my hand back in, I was bitten.
It was pure conditioning, Once a week she would get handled and then fed.
The day I handle but don't feed her, then stick my hand back in, I then get bit.
I've since started cleaning the snake cages while they're feeding in their feeding tanks.