Continuity mistake: During the scene where Lewis is first on the roof-top, and Mildred comes up to speak with him, the box he is sitting next to is plain wood on one side, and tallied on the other. When there is a close-up of Lewis' legs and the torn paper, when he's ripping the pages out of his book, the box has stencilled writing that is upside down.
Continuity mistake: In the scene after Lewis and Wilbur crash the time machine and are pushing it across a huge field, right before Lewis says "What about your parents?"the shadow of the time machine looks as if it's noon. In the next shot, when Wilbur responds "Mum never goes." the shadow is completely different.It's now to the left of the screen.