In the very first Fighting Fantasy book, The Warlock of Firetop Mountain, if you reach the battle with the titular Warlock and choose to fight him at his strongest, then he has SKILL 11 STAMINA 18. However, there are multiple ways of approaching this final encounter, including one that significantly weakens him to SKILL 7 STAMINA 12, and one where you can even bypass fighting him entirely. The book was co-authored by Steve Jackson and Ian Livingstone, but Jackson was responsible for writing the last battle.
Ten years later, though, Livingstone was the sole author of the sequel, Return to Firetop Mountain...
...and he remembered to give the resurrected Warlock the same statistics as his strongest possible version from the original book. A nice touch I'd never noticed before now.