ring gear teeth chipping off

[blueb8llz]

i replaced my diff oil last night and noticed that 3 of the ring gear teeth on my 3905 emaxx was chipped. i dont have a camera but, its 1/4 of a teeth and its the outer edge that broke off. two of the teeth was right next to each other, and the third broken teeth was like 8 teeth away and it had the smallest breakage. so what causes this anyways so i can be more careful next time.
wot from a standstill? shimming needs to be done? constant braking...?

and why didnt i notice anything wrong with the truck with 3 broken teeth? does it take more damage for me to hear or notice something? and what usually are the signs of broken teeth on a ring gear?