Continuity mistake: When Marge is speaking to the man behind the counter at Barney's Bowlarama, the word "Homer" can be seen above the three holes on her bowling ball. When the man says that she can't bowl without a lane, the word has disappeared, and in the shot after that, it is back again.
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.