The very end of Chapter 16 of the The Hitchhiker's Guide to the Galaxy says:
The deadly missile attack shortly to be launched [...] will result merely in [...] the bruising of somebody's upper arm[.]
[N]o revelation will yet be made concerning whose upper arm sustains the bruise.
The Hitchhiker's Guide To The Galaxy did this for episode three of the TV series, explaining which character bruised their arm in the attack. (This was the same stinger as in episode three of the radio series, from which it was adapted.) Done at the end of the chapter in the book, as well. The identity of the person who is bruised, however, is withheld at this point, since it was not particularly plot-relevant, and considered a safe level of suspense. As a result, the reveal is tacked on, seemingly as an afterthought.
However, searching through Chapters 17 and 18 of the book (which are the actual missile scene), I can't seem to find out whose arm actually gets bruised. TVTropes says that it was Arthur for the movie adaptation on a different page, but with the major discrepancies between every edition of THHGTG, I'd like to know if it's still Arthur in the book.
The closest I can find is the following:
It was of course more or less at this moment that one of the crew sustained a nasty bruise to the upper arm.
Does the book ever mention whose arm it is that actually gets bruised, or is it one of those dangling questions that Douglas Adams left hanging?