Continuity mistake: When the hamster Nibbles has rolled back to Skinner, it takes the lid of the ball off, and the lid lands on the ground. When the hamster is rolling away from Skinner it doesn't put the lid back on, it just leaves it on the ground, but in the next shot the lid is suddenly gone.
Continuity mistake: When the Hamster rolls through the glass of Flanders car it breaks it near the bottom middle of the front windshield. Later on when Flanders and Homer are taking the kids home you can see the break in the glass has now moved to about halfway up the front windshield. On of the following shots it moves back to its original position and then moves back to being around the middle of the windshield.