My Motorola Droid phone has a problem. The battery cover is too loose. The combination of a loose cover, rubberized backing on the phone, and my pocket means the cover frequently falls off.
I've seen a number of discussions in Droid forums about this, so I know I'm not alone.
Here's a very simple solution that I'm using on my own Moto Droid. The cover hasn't fallen off since.
And a little scotch tape makes it "permanent"...
UPDATE: Motorola will send you a replacement Droid battery cover.