When Bart shocks Lisa the first time, he uses the second button to his left. The second time he shocks Lisa, Bart uses the third button.
During the therapy session at Dr. Monroe's clinic, Homer draws a picture of an airplane in black and white. But during the scene in which the family uses the foam mallets, Homer's picture appears with color in the background.
When Marvin Monroe asks the Simpsons to draw the source of their anxieties, Bart draws with his right hand. The show repeatedly establishes elsewhere that he's left-handed, including the opening credits.
When we see Marge, Bart, Lisa and Maggie eating in front of the TV, they are shown eating with their left hands. But at the table they are shown eating with their right hands.
At the picnic, Homer is holding the gelatin when Bart insults him. In the next shot, the two aren't holding anything.
Marge counts the money in the college fund by increments of one cent, yet the three coins that she counts are silver.
When Bart, Lisa and Marge and eating their dinner at the dining room table, Marge's necklace turns to "white" in one of the frames of animation.
Homer is Drinking at the bar and says that his mom told him that he was a big disappointment, but his mom comes out later on the show as to be the chillest, coolest and supportive mom.
When Homer threatens Barney he says, "Here's five you haven't met" yet he has only four fingers.