Have there been any incidents in which an airliner would almost certainly have crashed had a passenger not taken some action (most likely telling a flight attendant that something didn't seem right)?
Its hard to prove a negative but there are cases where passengers have alerted the flight crew to things that caused a diversion or cancelation of a flight (arguably could have lead to accidents)
- This fuel leak noticed by a passenger lead to a cancelation
- This fuel leak noticed by a passenger lead to diversion
- This passenger (who had experience) was able to fill in for an ill first officer
- Joe Stiley arguably saved his own life by assuming the brace position with out instruction, before the fatal crash of Air Florida Flight 90. Being a pilot and flight instructor himself, he realized the aircraft was not moving fast enough on take off.
While these are seemingly benign examples, both fuel exhaustion and crew resource overloading has lead to crashes in other instances and you could argue, had the potential to in this situation.
Depending on exactly what your asking the passengers of United Flight 93 prevented a much larger disaster.
Also highly qualified pilots sometimes fly in the passenger role.
In United Airlines Flight 232, Dennis Fitch, an experienced United Airlines captain and DC-10 flight instructor, was among the passengers and volunteered to assist.
During this flight, the loss of all hydraulic fluid meant that control surfaces were inoperative. With one throttle in each hand, Fitch was able to mitigate the phugoid cycle and make rough steering adjustments.