Yes, thank you. I noticed that I needed to explicitly return true in the errornumber condition for my replacement string, or else it goes on to print the old "What?" after my replacement message.

That message is ParseError with error number 0, although that is one of the messages provided by the engine itself:
http://hugo.gerynarsabode.org/index.php?title=ParseError

Still, you can change it just the same using NewParseError:
http://hugo.gerynarsabode.org/index.php?title=NewParseError

Is there a way to substitute something else for the default "What?" printed when the player hits enter without typing anything? I notice that the "What?" string is neither stored in Message in hugolib.h nor VMessage in verblib.h. I don't know where it comes from.