My phone started that a good while back then the BB button started as well. I'm pretty sure mine was from dust or dog hair clogging the button mechanisms. Had to do an insurance replace last week after the lock and mute buttons stopped working, too.
If it happens in pretty much any app you're running, it's probably dust. If it isolated to an app or 2 it could be a new bug.
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Do you have the phone in a holder that compresses the side buttons when this happens?
My side buttons are set to do nothing, but when pressed in my car holder I get weird stuff going on
Ie if I set a song playing then try and go back to the album track list or further back to the artist list it won't, it'll let me go all the way back to home screen tho if I press and hold it, just not 1 step at a time!
Can be reproduced without using a holder, its an idea anyway eh!
Posted from my CrackBerry at wapforums.crackberry.com