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.
Other mistake: At the start of the song, Iceland appears twice on the map, and in two different colors. Also, a large portion of Antarctica is cut off.
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 Vinnie Jones takes the King to his room, his earring keeps changing from ear to ear between shots.
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).
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.
A Day in the Life of Retsuko - S1-E1
Continuity mistake: After Ton bashes Retsuko for not getting her tea right, she sings some death metal in the bathroom before joining Fenneko back at the computers and they start talking. Between them is a pink cup with some orange scissors, and an assortment of writing utensils. They are fanned out, but show that the red one is closed to Retsuko with the black in the middle and the yellow closer to Fenneko. When the camera changes angles to show them both sitting there, this order of pens is flipped, with the scissors still being in the same spot.
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.