Every now and then while reading programming books, I find more a less hidden jokes, passage of texts written in witty, tongue-in-cheek fashion, right there in middle of more serious text. These make me giggle and I remember these for years :)

Such as

• The Java Programming Language, documentation about currentTimeMillis(): "The time is returned in a long, so it will not overflow until the year 292280995, which should suffice for most purposes. More sophisticated applications may require the Date class."
• Common Lisp The Language, 2nd ed, in index: "kludges, 1-971"

What good "hidden" jokes you have come across while reading programming books?