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: The mice on the table are in slow motion. What's more, you see the same shots of them several times.
Revealing mistake: Right after we see the two suns of Magrathea, the screen fades to another scene. As the screen fades, pay close attention to the sun on the right. You will notice that the "sun" is actually a light bulb that turns off right as the screen changes shots.
Chosen answer: That seems to be an error in the subtitling of the episode. What Marvin actually says is: "I may just be a menial robot, but I'm far too intelligent to expect to think of me for a moment... far too intelligent..."
Sierra1