Critical Research Failure: One volume had, following an article on the history of Family Feud, a list that was ostensibly funny answers culled from the show. In actuality, most (if not all) came from the British version, Family Fortunes. You'd think the fact that one of the questions was "Name a job that requires a torch" (Britspeak for "flashlight") would've tipped them off.
The same book had a list of trivia about sloths. In the first printing, it mentioned a species of sloth that was apparently dying off because it took too much energy for it to mate. This "fact" was replaced with another in later editions.
Growing the Beard: Volume 8 (Uncle John's Ultimate Bathroom Reader) which was larger than the last two volumes combined. From this point onward, the writing became gradually more and more in-depth, eventually leading to an "extended sitting" section composed of even longer stories, as well as a large number of multi-part stories spread throughout the books.
Hilarious in Hindsight: The "(Not) Coming to a Theater Near You" section in some of the books lists concepts for movies that were never made. In the "Slightly Irregluar" Bathroom Reader, it contains a description of what could have been for the film Avatar.