I often wonder how many authentication log files contain passwords because people in a hurry append it to the username on accident (not visually confirming the Tab/Enter/switch to the password entry).

This is also vaguely similar to the 'test SSL submit' security technique of first entering enough data into login forms to process a submission, and then entering real login info into the 'login failed' retry page after verifying SSL. This has lost some of its luster as non-SSL form submission has fallen out of wide usage.