When Peter is playing the piano in the bar the amount of beer in the glasses of his friends changes from full, to empty, then back to full within a few seconds.
When Pawtucket Pat opens the beer bottle door handle to the beer room, beer fizzes out from the bottle. When the door is opened and viewed from the other side, the bottle handle splits into two solid halves.
In the scene where Peter is playing at the piano competition, Lois pushes him towards the lower end of the piano, but the pitch of the notes rises - it should actually do the opposite as the keys at that end would play lower notes.