I nearly left one in once....
Ahh the 1-11, that takes me back.
I had been sent out to refuel a 1-11 ready to be a 'backup' in the days of Shuttle (sheds a tear).
The a/c was on one of the 'November' stands, I think it was N41. As the fuel went on I did a walk around and took the nose gear lock out and put it on the flight deck.
The a/c was then 'stood down' and I returned to the crew room. Half an hour later, I was then told 'it's back on' can you go and depart the aircraft. So off I go, did another quick walk around and shut the hold doors all ready to push.
As I sat on the tow bar I noticed something just poking out of the nose u/c bay. It was the flag on the lock!.... quickly removed and delivered to the flight deck just before the doors were shut.
I was somewhat confused as to how it had been fitted, only to find that in the 30 mins I was in the crew room the a/c had been moved to the next stand (not sure why) and the towing crew had put the lock in.
I have gone to the a/c and located it by it's reg and not the stand, as there were so many 'November' stands in a line it was easy not to spot it had moved.
There but by the grace of God go I...