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.
Continuity mistake: In "Wheels," when Artie is singing 'Dancing By Myself' and doing tricks in his wheelchair, the wheels on the back of the chair that prevent him from falling backwards disappear and reappear, depending on whether or not he is performing a trick.
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: 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: 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.
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.