Book In-Jokes in...
Computability: Computable Functions, Logic and the Foundations of Mathematics
by Richard L. Epstein and W. A. Carnielli
(Published by Wadsworth Publishing, 1989)
In the preface:
Each author wishes to indicate that any mistakes still left in this text are not due to those above who have so generously helped us, but are due entirely to the other author.This entry was mailed to me by one of the book's authors, or the other.