Revealing mistake: During "A Man's Gotta Do", when Captain Hammer kneels down and crushes the gizmo on top of the van, you can briefly catch a glimpse of a black safety cable next to his knee.
Dr. Horrible's Sing-Along Blog (2008)
1 revealing mistake in show generally - chronological order
Starring: Neil Patrick Harris, Nathan Fillion, Simon Helberg, Felicia Day
Continuity mistake: During the very beginning of "My Freeze Ray", as Dr. Horrible is about to set down his basket of laundry, the bottle of detergent sitting on top of his clothing rotates 90 degrees after the scene briefly cuts to Penny.
Moist: I'm Moist. At my most badass, I make people want to take a shower.
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.