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.

Trivia: After the series became a major success upon release in 2008, director Joss Whedon teased that he would be doing a sort-of "sequel" at some point in the future. At one point, an outline and a number of songs for the sequel were written, though due to other obligations, Whedon had to push back production. As of 2018, the long-rumored sequel has yet to materialize, though everyone involved has continued to hint that a follow-up may still happen eventually.