Continuity mistake: When Oliver falls into the fountain with the wooden crate, you can tell the crate is empty because it is floating. But when we see a close-up of Oliver in the next shot, the crate has been weighed down and is not floating.
Continuity mistake: When Stan pokes Oliver in the eye, Oliver steps backwards and you can see the floor around him is saturated with water. But when we see Oliver putting his shoe into the nails on the piece of wood, the floor is now completely dry.
Visible crew/equipment: When Oliver starts walking down the steps to see the policeman, the piano crate starts sliding down the steps after him. You can see the shadows of the camera and crew on the side of the crate when it slides past.