I noticed that one of my overhead fluorescent fixtures was dim, so thinking that a bulb had burned out I pulled the plastic cover and a bulb -- as well as some plastic bits -- fell out
Short story is that there are 4 rivets that attach the fixture sides to the base and one was missing. Also, the fixture was secured to the ceiling with only 3 rather than 4 possible screws. The combination allowed one tube to vibrate enough that the plastic socket covers for the tubes eventually failed.
I ended up replacing it entirely with LED's.
The long version of this is on my blog
if you're interested.