He added a fire emoji to one success message. When testers saw it they were afraid that the customer would think it was a thermal runway problem. Had to do a last-minute revision of the software before shipping the new version.
I was already pretty anti-emoji / personal touch / fun features / easter eggs in professional software. But having to pull a 2-hours overtime to crank out a new release definitely settled me on the side of never again.
edit: To be clear no one actually thought it was a problem, but our QA were very much serious about reducing any potential for confusion when dealing with >1million USD machinery.