Continuity mistake: When Ernest picks up the Christmas tree from the freeway, he throws the Christmas tree into his taxi. It smashes the driver-side rear window. But, when he drives away, that window is still intact.
Continuity mistake: The first time we see Ernest driving the cab the passenger pats his shoulder telling him to hurry up. The passengers hand was on his shoulder when he said this but in the next shot both his hands are on his briefcase.
Revealing mistake: At beginning of the movie when Ernest is taking the guy to the airport and his car jumps the bump in the road and goes in to the air, sparks fly from underneath before the car touches the pavement.
Revealing mistake: When Ernest jumps the luggage trying to get the airport, sparks fly from out of the car. If you look, they aren't coming from where the car is hitting, but a spark generator on the front of the car.
Visible crew/equipment: At the end of the film, when Ernest comes in with the sleigh, the camera moves down to ground level as the actors on the ground are shouting at him. When the camera is descending, it creates a shadow on several of the characters as well as the ground.