The old saw is that the last things you want to hear on an airplane is the pilot saying "Hey, watch this."

I think after reading this excerpt from Planecrashinfo.com's Unusual Crashes page, I'd add to the last thing you want to hear would be a jump-seat pilot snickering behind you:

As a prank, a captain riding in the jump seat engaged the gust lock in flight. The command pilot, not knowing the gust lock had been engaged, rolled the elevator trim tab with no response. When the jump seat  captain disengaged the gust lock, the aircraft went into into a steep dive, executed part of an outside roll and become inverted. Neither the command nor jump seat captain had seat belts on and they accidentally feathered No. 1, 2 and 4 engines when they hit the controls with their heads. No one realized it at the time but the feathering reduced power and allowed the co-pilot, who was strapped in, to pull out of the dive 350 feet from the ground.