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 Bill is pushed out of the helicopter and Ralph rescues him in mid-air, its obvious in some shots that it is two sky-divers dressed as Ralph and Bill. Firstly you can see the parachute back-pack bulges on both men. Also the wind is blowing up on Ralph even though he isn't meant to be descending.