password not recognized!

This is a first... my iMac suddenly doesn't recognize my password. It's denying me access to anything admin related. It's been the SAME password the entire time I've owned the @#$#$ thing, and each and every time I reinstall OS etc - now suddenly in the last 15 minutes it changed its mind....

What's going on here? and how do I work around it?
have tried disk utility to repair permissions... it says all's well...

 

So, that was wierd, and a bit scary for a while there....
boy, very 2001-ish, when HAL takes over control - you don't have access priviledges anymore...

Um, not sure what the problem was, it appears to have been some sort of glitch in a software update. I shut down, restarted & had to re-register everything, but it recognized me as lord & master again.

For future reference, though, what would happen if one maybe say lost or forgot a password? is there any way to unlock it if it doesn't like your password?