II (The Samurai called Jack) - S1-E2
Revealing mistake: When Jack cuts the flying car's hood off, he slashes it vertically, but it is cut horizontally.
XX (Jack and the Monks) - S1-E20
Revealing mistake: Immediately after the time portal is destroyed, it stops raining but there is no moisture anywhere.
Chosen answer: Gotta get back, back to the past...
Rlvlk