Ed Holland

I've certainly had both cases of bad bearing:

1) Insufficient protection from the elements - resulting in corrosion, and abrasion and ultimately pitting and disintegration of the balls. This was a Sora hub set.

2) Over tight adjustment on a new wheel set with Ultegra hubs - front wheel bearing started making strange ticking noises and on inspection, broken fragments of ball bearing were found in relatively clean grease.

Ed