I was on watch at an airport outside the UK once when all of us in the tower observed a complete wheel come off an airliner whilst it was taxying for take-off. It took a lot of effort to convince the crew to stop, believe me! Later we heard that a pax had seen the wheel rolling out from under the wing, pointed it out to a cabin crew member who said: "don't worry, sir, everything's fine". I still wouldn't fly with that airline, 30+ years later.