A time you've been asked to implement something you felt uncomfortable with.

Had to use MD-FUCKING-5 as hashing algo WITHOUT salts. That felt very wrong

@linuxxx I'm so sorry... <3

Registration & contact forms that added users to a newsletter mailing list regardless of the "yes, send me a newsletter" checkbox.
I expected it to be an oversight/bug and "fixed" the condition.
Boss got very angry when he found out and shouted through the office why I think I have the authority to change that, as we had a 90% drop in newsletter registrations that month.

Protip: don't kidnap your users.