Peculiar password problem

Do you have a USB or bluetooth keyboard you can use to attempt login? My first thought is a faulty key. (SHIFT or if there's a letter that's in the passwords you're using but not your username..?)

Maybe http://support.apple.com/kb/ht1379 is worth a try?

My first thought was caps lock or number lock was on or off, whichever one you don't expect it to be.

If you can get to the console, and sudo works, then can you https://apple.stackexchange.com/questions/82472/what-steps-are-needed-to-create-a-new-user-from-the-command-line-on-mountain-lio and test the GUI with that new user?

We once had a somewhat similar problem to this at my office. It turned out that the person had previously paired a wireless keyboard to his machine, but wasn't using it any more. The keyboard was sitting in a corner on someone else's desk. Somehow it got turned on, and it was still in range, so it connected to the original user's computer. It had a bunch of junk piled on it, which caused it to constantly send a stream of spaces, which was very baffling until we figured out what was happening. I could see a similar thing happening with shift or caps lock, which would be less immediately visible, but could cause problems like this.

Never seen this on the particular issue, but keychain first aid? Although I think that might need the admin password...

There is no wireless keyboard connected to this computer. I used the same USB keyboard to (successfully) enter my password at the command line as I did to (unsuccessfully) enter it in the GUI. Apple keyboards don't have a Num Lock key, and yes, I made sure Caps Lock was off. I'll try creating a new user and see if that works.

when it tried to use my newly-reset password to automatically log in, it was rejected again Perhaps the system and/or login components of your Keychain are corrupt? If there's any way to get to the GUI via another account, you might first give Apple's http://support.apple.com/kb/ph7296 a try, via the Keychain Access utility. If you have a backup and can get to root via CLI, you could also run the command-line equivalent to list keychains, e.g.: $ security list-keychains "/Users/foo/Library/Keychains/login.keychain" "/Users/foo/Library/Keychains/Microsoft_Intermediate_Certificates" "/Library/Keychains/System.keychain" You could rename the login.keychain, then do touch /Users/.../login.keychain to create an empty file. Then try logging in to your old account again, in order to repopulate a fresh login.keychain. If this fails, because you renamed the original file, you can cp it back to its original location. You might also wipe the System.keychain file. To create a fresh System.keychain you could use /usr/sbin/systemkeychain -C -f when logged in as root. I don't think you can just delete this, as it relies on another file for validation. Setting up fresh keychain files may force setting up authenticated services again, once you're logged in. Once again, make sure you have a backup before you do any manipulations of these core files.

Do you have the computer set up for multiple languages / keyboard layouts? I use both Dvorak and QWERTY, and sometimes certain OS password panels will have a different keyboard selected. In some situations the panel has a menu tucked away in a corner where you can change the keyboard, but I've also seen situations where that isn't the case and you just have to guess and try the other keyboard. (This is basically a longer version of what Gungho suggested.)