Revealing mistake: In the scene where Doc Brown is following Biff into the tunnel, as Marty starts to pass between the cars on his hoverboard, one of the wheels on the rig supporting the hovering DeLorean is visible. (01:30:30)
Revealing mistake: When we first see Max, his paws are leaning on a balustrade and become transparent. Though it may seem like it's an optical effect because they're in the shadows, the next angle shows them fully lit and opaque.
Revealing mistake: When Nick is being sucked out of the hole/cave, you can see the wires visible from his waist. (01:20:35)
Revealing mistake: When Santa starts reciting the long list of things the little boy wants for Christmas, you can see the boy's eyes reading Santa's lines off-camera.
Revealing mistake: Near the end of the movie, when Wallace says: "hold tight lad, and think of Lancashire hotpot" one picture of a sheep on the ceiling slightly moves a few times. (00:19:57)
Revealing mistake: The time on the scoreboard clock when Ray hears the "go the distance" message changes from 10:30 to 10:32 in a matter of seconds.