Dr. Horrible's Sing-Along Blog (2008)

Act II - S1-E2

Revealing mistake: During "Brand New Day", watch the background as Dr. Horrible appears as a giant and stomps down the city-street. As he takes his first step, a piece of concrete from building to his right falls off and hits the ground. However, if you look closely, it's obviously a quickly-made computer effect. As the piece of concrete hits the ground, it simply vanishes, and is replaced by a rather odd-looking animated "poof" of dust. The issue is the concrete piece was much bigger than the cloud of dust, so it's clear that the piece simply disappears right as the dust-poof appears.

Dr. Horrible: Oh goodness, look at my wrist, I have to go.

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.