Continuity mistake: When Homer says that Marge hasn't opened his present yet at her birthday dinner, there is a plate and glass in front of him on the table. But then the waiters come over to the Simpsons' table in the next shot, look at Homer and you can see that the glass and plate have disappeared.
Continuity mistake: When we see Homer eating at Marge's birthday dinner at Singing Sirloin, there is a cake on a dish and a wine glass by his plate. When we see Homer a little later on, the cake and wine have disappeared and there is now an ordinary drinking glass by his plate.
Continuity mistake: When Mr Burns tells Homer to wreck something, Homer throws a plant pot onto the floor and it breaks into several pieces. When Mr Burns tells Smithers to take him home, the pieces of the broken plant pot are nowhere to be seen.