When Bart looks through the mail slot the delivery man has yellow skin and is wearing a light brown shirt and dark brown pants. But when Bart steps out he has darker skin and is wearing a blue jumpsuit. He still has the darker skin color when he punches Bart but in the next shot he has yellow skin again.
When Marge is on the phone with Jacqueline, Grandpa Simpson picks up on the upstairs phone to yell "I love you!" to Jacqueline, and Marge ends the call by hanging up. On a land line both phone would need to hang up to end a call.
When the family is reminiscing about Lisa's third birthday, before Bart and Lisa do their old song and dance routine, Maggie's birthday cake is gone. In the next shot, when the adults are dancing around the table, there is a full cake again.