I had a bunch of misspelled logins saved for a site where I couldn't remember the exact "organisational username" for my organisation. Every time I went back to the site I was asked to choose between them (though not all of them, curiously), and of course I couldn't remember which one was the right one. Not too hard to find them and delete them, once I realised there was an option to do so in preferences dialog, but it would be nice to have this small feature: If a server rejects a login (this is unambigious when it's a HTTP AUTH login, not so easy to determine with web-based logins), a dialog would appear saying something like "Server seems to have rejected your login (username: xxxxx). If you misspelled this username, you can choose Don't Remember and Password Manager will not remember this username." with buttons: "[Don't Remember] [Remember Anyway]" The choice should be presented just in case the login failure is due to a server bug rather than user error. Misspelled passwords don't matter nearly so much because they will be overwritten when the correct password is entered.
You don't need to go to preference dialog to do the deleting. It's more obviously found in the password manager dialog itself. I'm pretty sure that this rejection is already being done for http and ftp authentication. If it isn't, then there's a bug in the http/ftp code and a bug about that should be filed against the networking group. Regarding your comment about it being not so easy to determine with a web-based login, I would say that's an understatement. There really is no practical test that I can make to determine that a web-based login failed. That's the reason such rejection had not been built in to the password manager already. Therefore I have to mark this bug as wont-fix.
Agreed, marking verified