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.]
Reseda Rose - S1-E6
Revealing mistake: During the meeting on the beach, while Bill remains on the hill Ralph flies to the surfaced submarine, and in Ralph's closeups on the sub the crinkled black set wall behind him is painfully obvious.