Other mistake: During "My Freeze Ray", there's a shot where Billy, Penny, and several other people open doors to the laundry machines in sequence. When Billy opens the door to his machine, you can see the reflection of everyone else just standing there, waiting for their cue to walk forward.
Dr. Horrible's Sing-Along Blog (2008)
1 other mistake in season 1 - chronological order
Starring: Neil Patrick Harris, Nathan Fillion, Simon Helberg, Felicia Day
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.)
Dr. Horrible: I don't have time for a grudge match with every poser in a parka.
Trivia: Was originally meant to be shot entirely with web-cams, with the characters reflecting on the events through dialog in video-blogs. However, after the story grew bigger during the writing process, it was ultimately decided that it would be filmed like a more traditional film or series, and the video-blogs became more of an occasional recurring motif than a main focus.