Continuity mistake: When Stan pulls the door into Oliver's face, Oliver falls backwards and the spring on his back sends him skywards and onto the piano. When Oliver falls onto the piano, you can see that there is nothing on his stomach, but a few seconds later he is covered with piano hammers and other parts from the interior of a real piano.
Continuity mistake: When Bosko is sitting in the trench, he spots a piece of cheese on a mousetrap and eats it. In the next shot, the mousetrap has disappeared.
Continuity mistake: When Stan and Oliver are speaking to the ice-cream salesman, the amount on the till reads "05", but after Oliver has given him the money to pay for the ice-cream, the amount is now "10."