Towards the end of the movie when Biff is getting ready to head back into the tunnel after Marty. When he is reving the engin, the camera flashes on the license plates and it shows 1954 and then a second later it shows 1951.
Obviously they would realise they got the year wrong and would've changed it before filming. The cast and crew must have been asleep. You did a good job at spotting it because I didn't.
It is clearly 1951 both times. Because of the angle and lighting in the first shot, it may have looked like 1954, but I guarantee that if you clean the dust/dirt off your TV Screen and check again, you'll see both times it shows 1951.
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