The ending the sticks with me the most is Inside UFO 54-40. There's a specific ending you cannot reach by following the instructions. You have to "hack" the book and turn to the page directly.

Ah, I was going to post asking if anybody else read all the pages looking for unreferenced endings.

That was intentional, at least.

What surprised me was that there were actual bugs in some of the books. For example, some editions of "Vampire Express" have a typo that leads you to the wrong page, breaking some paths of the adventure.