My Babies used to stalk my fingers, they never bit me because I made sure they never had the chance.
Spaz got me one time, and like you said it din't hurt it just scared the heck out of me. I don't know if there is any way to avoid this. It hasn't happened to me again, yet. Where you moving your fingers around or anything? Movement tends to trigger their feeding response, exspecially in the little ones. Try to an eye out for any predatory behavior while your fingers are in their tank, but they might still launch a surprise attack. I think it was more than likely a case of misidentity. What was in the bowl? Have they eaten out of the bowl before? Just asking, because mine completly ignored the bowl of mealies I had in their tank util they realised there was food in the bowl. The bowl sides might be too high for them to see in. Hope this helps.
-----
Mommy of 6 Leopards
Milo(my male) Mongo, Raptor,and Spaz
My babies Popeye and Boo(named after Monster's Inc. Character)
2 Dogs
Baby a Peekapoo and Meshu a terrier mix
And a Cat named Mr. Kitty