Continuity mistake: When Kelly is putting the number card on Logan, you can see one on Kendall behind him, when he shouldn't have had one yet.
Revealing mistake: Season 2, episode 4 (Duets): When Brittany lifts Artie from his chair and carries him to her bed, right before she puts him down, he is obviously lifting his feet to prevent them from bumping into the bedframe.
Continuity mistake: Bob Fossil records Howard saying "Gorilla" on his talk box, which Howard does with a downward inflection at the end of the word. However, every time Fossil plays back Howard's "Gorilla", it has an upwards inflection at the end of the word.
Continuity mistake: During "Brand New Day", at one point, Dr. Horrible stabs a picture of Captain Hammer attached to a dartboard with a handful of darts, all right in the center of the picture. A few shots later as Dr. Horrible sits, suddenly the darts are far-more spread out. (It's in the background and can be hard to see, but it's noticeable.)
Continuity mistake: After the Squirmese chef sets up his additional ingredients, Corky answers the video phone. During Corky's conversation, Al's kitchen can be seen with no trace of the chef or his ingredients.
Continuity mistake: When Yakko points to India, Africa, except for Egypt and Madagascar, is shown as one country until he points to Tunisia.
Continuity mistake: When Vinnie Jones takes the King to his room, his earring keeps changing from ear to ear between shots.
Monster in the Playhouse - S1-E9
Revealing mistake: As the monster hops from behind the snack island, the actor's white shoes can be seen for a moment. (00:30:00)
Revealing mistake: When Mr. Goodman blows his nose, you can see a little pink from the makeup on his nose (to make him look sick).
Continuity mistake: At the diner when Tsunoda and Retsuko are talking, Tsunoda pulls out a red phone with a white heart on it from her purse. In the close up she is pulling it out with her left hand and the phone is horizontal with the heart on its side. It then cuts to a wide angle and suddenly she's holding the phone upright with both hands.
Lisa's Song - S3-E14
Factual error: Doris finds out that Gypsy Queen "Lisa" died from influenza around 1918, so she didn't get to star in her performance. However, when Doris and Lisa's ghost are singing their duet later, Lisa's hair and costume are 1980s style, instead of the era she died.