Only once did we abandon a gate--10 mins before departure:
the agent had just paid off two rev pax to fly next day--hotel vouchers etc.
We went to our hotel and saw the flight went out with 4 empty seats but 2 S4's got on. That was a thousand dollar error on a seat count!!

The trickiest are the commuter airports like JFK/LGA and ATL--no shows and last minute shows. Even travelnet can't be accurate in those circumstances!