Safari was storing an old password for my Gmail account, so it kept pre-filling an incorrect value.

It didn’t let me delete the obsolete entry, though. I tried clicking “Remove”. Nothing happens. “Remove All”. Still nothing.

Even when the checkbox to auto-fill usernames and passwords is empty (unchecked), the form still gets auto-filled!

Resetting Safari didn’t help, either.

The answer: Keychain Access.

Once you know that you have to use the Keychain, everything seems to work fine.

Keychain Access is located in Applications/Utilities.