You enter the user name and password to log on a web service. Firefox asks you if you want to save your credential and you accept. Then you realize you mistyped your username or password but now Firefox has saved it and you will have to either select the correct one from a list every time you access the same web site (if you are lazy) or will have to manually delete the wrong credential (press the down button in the user name field, select the value you want to delete and pres Shift + Delete).
This wouldn’t have happened if Firefox offered to remember a password after the log on attempt so you can more properly decide if the credential is valid and you want to save it or not. And that’s what Firefox 3 will do starting from the latest development release (nightly): after a log on attempt, the information bar is prompted asking if you want to remember the password.
It is not smart enough to detect if the logon was successful but it will certainly help to less cluttered and efficient credential lists.