I couldn't log in today. Weird. Turns out I left a space after my username in the login field.
Somehow I remembered that Drupal will trim your password of whitespace before looking it up.
Just confirmed it. You can put as many spaces as you want after your password in any Drupal login form.
But if you do it to your username? Baffled. You've stumped Drupal.
It's kinda fun.