Revealing mistake: At the point where someone gets a small bruise on the upper arm, the crew are all in a pile and thrashing about on the floor. There are 5 pairs of legs instead of 4. Arthur is in his pajamas, Zaphod is in orange trousers, Trillion is lying on top of Ford and Ford is too far away for them to be his legs. [The fifth pair of legs belong to the person playing zaphod's third arm - doesn't mean it's not a mistake, but that's where they come from.]
The Two-Hundred-Mile-an-Hour Fastball - S2-E1
Revealing mistake: During Ralph's tryout for the team, when the catcher yells, "Come on, skinny bones, try to put one right there...," the stunt wire is visible behind the catcher in this shot as well as the following shot, when he flips over. (00:10:10)