I've had it happen a few times. It tells me I'm using the wrong password. I'm not but I'll the message a few times in a row as I try it again and again. Then the next time it works. This is all in one session of trying to log in to mail.
MBP 2.66 Ghz Intel Core 2 Duo, OS Snow Leopard, FCS
G5 dual 2.0 Ghz, OS Tiger, FCS (contemplating the closet
G4 dual 500 hz, OS Panther, FCE2 (sitting in closet