Do as I say, not as I do: Ciro Santilli's Stack Overflow suspension for vote fraud script 2019, meta.stackoverflow.com/questions/381577/is-it-ok-to-have-links-on-how-to-create-sock-puppets-and-gain-rep-fraudulently-i/381635#381635.
Basically the opposite of security through obscurity, though slightly more focused on cryptography.