Continuity mistake: After Wallace gives the woman the balls of yarn, the balls change position between shots.
Continuity mistake: When Wallace is put into his overalls he's still wearing his green pullover. A moment later he suddenly doesn't wear it anymore but a complete white shirt. He couldn't have just taken off the pullover as the shirt under it is actually fake (The collar is actually part of the pullover and the arms are just stuck on).
Continuity mistake: The oatmeal machine fires oatmeal at Wallace, and some of it lands on the wall. In the next shot, the porridge is gone.