Continuity mistake: When the construction worker sees a bird in front of him, he says "Ha!" The worker then sees the city below and falls. At this point, you can see the worker is holding his hat in his left hand, but when he falls through the first plank of wood, the hat is nowhere to be seen.
Continuity mistake: When we see Sylvester reading his newspaper at the start of the cartoon, you can see the front page has only one picture on the cover, but when we see a close-up of the picture in the next shot, another picture has appeared under it.
Continuity mistake: When Tom shoots a stream of balls to flatten Jerry, the last ball in the pack is coloured purple, but when the balls rebound off the side rail, the last ball is now a green one.