Some Sierra-on-line adventures required the manual in order to know the proper procedures your character was supposed to follow (protocol to board a ship in "Iceman", for instance, maybe some police things in "Police Quest" too).
Some LucasArts had a similar thing. The manual was required in order to open a safe door in "Maniac Mansion", or to fly the plane and to identify the Grail in "Last Crusade".
Opening the hatch in "Infestation" (an interesting Amiga game) required to know the main character's name, which was only mentionned in the manual.
And "Wasteland" (as well as some other game from the same company) required some descriptions paragraphs to be read from the manual.
So yeah, all the exemple that come to mind are questions about the manual, more or less well integrated into the game's story. Sometimes it was a bit more heavy-handed, like a character asking you for the first letter of the fifth line of page 28, under one pretext or another.
Ah also the manual was required for the magic recipes in "Ultima IV", so, there's that sort of documentation-based protection.
(And just remembered that "Rocket Ranger" travels by filling your rocket with the exact amount of fuel needed between two countries. The amounts were listed in a table in the game's documentation.)
Post edited July 03, 2018 by Telika