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.
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