Other mistake: In the opening song, the dolphins sing: "If we could just change one thing, we would all have learned to sing", however the subtitles on the DVD say "lungs to sing". As a side note, dolphins do indeed have lungs.
Continuity mistake: When Arthur breaks the arm of the chair as his brain is being taken, the arm is strapped to his wrist. A second later the strap and broken arm are gone.
Slartibartfast: You must come with me, quickly or, or you'll be late!
Arthur: Late for what?
Slartibartfast: What? Oh, no...What is your name, earthman?
Arthur: Dent. Arthur Dent.
Slartibartfast: Late, as in the late Dentarthurdent. It's a kind of threat, you see.
Trivia: At the very end of the film, when the group uses the Improbability Drive to go to the Restaurant at the End of the Universe, there are a lot of images that flash onto the screen when the drive is activated, so in the other parts of the film. In this case, the very last image shown is the face of Douglas Adams, the creator of the Hitchhiker series.
Question: Anyone who's read 'Life, The Universe & Everything' will now why the bowl of petunias thought 'Oh no, not again' just before it plummeted towards Magrathea, but had Douglas Adams already conceived that backstory and just couldn't fit it in to the narrative at the time or was it something he only came up with when writing part 3?
Chosen answer: Because it was a radio show originally, Douglas Adams would write broadcasts from show to show. He did not know where something would lead, which let the plot become random and funny. So ultimately it was originally created to be funny, then he found a way to link it back in later.