Continuity mistake: There is a scene where Hugo is on the station and sees a key down on the tracks. There are two seperate shots of the key and both times it is buried in the stones next to a sleeper. Then Hugo jumps down onto the tracks to pick it up and its now quite sat on the middle of the sleeper when he picks it up.
Continuity mistake: When the automaton starts writing it stops the first time with the pen tip over a small figure made of three blocks. The shot switches to a rear view of the automaton and its elbow moves all the way back to the starting position. The shot changes back to the pen tip and it is still over the small, three-block figure. This is impossible as moving the elbow all the way back would have moved the pen away from the drawing.