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.]
Revealing mistake: When Edward opens the gate to let Tracy in, we then see a shot of Tracy and you can see it is repeated footage of her from when she met Edward for the first time.
Heaven is in Your Genes - S3-E6
Revealing mistake: While Ralph is in mid-air, en route to the airport cafe, something dips into view at the top left corner of the screen. Then when Ralph takes off from that cafe, two shots later as he's in mid-air, we can see the shadow of the helicopter filming the background stock footage, but it's been flipped so the helicopter is flying backwards, and when more background footage is shown again the helicopter shadow is flying properly. (00:13:15)