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 the Warners emerge from a sealed crate on Miss Flamiel's desk and start singing, Yakko instantly appears behind Wakko and Dot as soon as the shot changes.
Continuity mistake: When Vinnie Jones takes the King to his room, his earring keeps changing from ear to ear between shots.
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.
Monster in the Playhouse - S1-E9
Revealing mistake: As Reba the Mail Person exits the Playhouse, she goes past the window, and you can see her exit the sound stage through a door painted the same as the background.
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).