Which Browser? Im guessing it could be that the entries have been saved/remembered as passwords by the browser. In firefox 3 you can view and remove them. tools>options.security>saved passwords>view saved passwords and remove the rogue entries leaving the correct one in place
I was thinking of my o2 login page: I originally put in my full email address instead of just the bit before the @ sign. I've also got the same problem with the box on our club website: the password is changed now and again, but all the old ones are listed.
With my memory, it's a bit trial and error picking the latest one!