First trip with my partner after meeting him two months before. We were flying BOS-SFO. Used the last of my miles to upgrade us because it was my first trip with him and wanted it to be cool.
We get on the plane and it pushes back. Taxiing out, I take off my watch and turn it back three hours. He looked at me with this horrified look. "Don't ever do that until wheels up!" (his father was a United pilot for 30+ years).
We're now second in line for take off. Captain comes over intercom. Issue with plane. Turns around, we go back to gate, we all get off the aircraft, and ten minutes later they announce the flight is cancelled.
We're standing in the gate area and looks at me he says: "See what you did!"
I was mortified. He was cracking up laughing.
Lesson learned.