coolest bug our team had was not a actually a bug but a feature that is misused and abused.

tldr: its a feature that became a bug

we have an app that has a "test print" feature to test the printer and the format of the document to be printed. it has the word TEST for fields and all that.
it became a bug when suddenly, the users use that feature to print documents, instead of using the app with the business rules and all, and just manually strike off the TEST words with a pen.
the feature became a bug because it has become a security risk.