Continuity mistake: In "Broke 'N' YoYo", when Cody has just sat down to talk to Zack after collecting loads of towels, in the first shot the towels are on the chair in front of him. In the next shot, when Cody leaves to practice, the towels have vanished. And in the next shot, when Bailey and Zack are talking, the towels are back again.
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.)
Revealing mistake: In the scene where Mitchell first visits McNair's van, Mitchell's reflection is visible in the window on the left side of the screen during the shot where he first opens the van door. The vampire lore of the show informs the viewer many, many times that vampires (such as Mitchell) and ghosts cannot be seen in reflections, pictures, or videos (their image "cannot be captured"). The reflection can be seen for roughly 20 seconds. (00:23:20)Ravenicus