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Almost all of Sierra's point-and-click adventure games had copy protection in their manuals, meaning that those who used illegal copies of the game (or who just plain lost their manual) couldn't progress any further. Most of these copy protection checks occured when starting the game, although a few subverted this trend by placing them about halfway through the game. Some of Sierra's examples:
Codename: ICEMAN: The game begins as the main character is on vacation in Tahiti. A nearby volleyball player drowns in the surf and the player must rescue him and perform CPR. Obnoxiously, the game didn't tell you that it wanted you to look in the manual and type off the instructions verbatim. However, the introductory walkthrough in the game's manual offers step-by-step instructions, making this section trivial for legitimate first-time players.
Conquests of Camelot: The Search For The Grail also used this system - you had to look in the manual to solve various riddles throughout the game (but you learned some interesting mythology in the process).
The Even More Incredible Machine required you to look into the instruction manual to input a code on a randomly decided page each time you opened it. However, during the game's intro, if you clicked to get past it at just the right time (specifically, when it switches from the second screen back to the first) it would almost always request the code on the first page of the book, requiring you to remember only one code.
Freddy Pharkas: Frontier Pharmacist requires that you look up recipes in the enclosed "home health manual" and create the prescriptions to solve certain puzzles. Only problem is, when the game was re-released in the Sierra Originals version, only a truncated version of the manual was included in the CD booklet, and one of the required recipes was left out entirely. Oops! Al Lowe, the game creator, has since put the entire doc on his website.
A certain line of code in the King's Quest Collection, which included games I-VI, had a misprint in it, leading to a player most likely getting the spell wrong until they noticed that the misprinted manual decided to rhyme "thither" with "thither" instead of "hither". The VGA remakes with the copyright stripped out that allow the player to just work the entire spell with a single command actually make the game vastly easier.
King's Quest III: To Heir Is Human: A very large part of the game revolved around copying lengthy, exact instructions for magical spells from the game manual. Getting the instructions even slightly wrong would end the game. The correct phrasing was to simply type over the exact sentence in the manual, although words like "the" and "a" could be omitted. This was in addition to the disk check at the beginning of the game (that all Sierra games had at the time).
King's Quest IV: The Perils Of Rosella actually has a copy protection joke inserted into the coding. Activating the command box at the start (Ctrl + D) and typing in "Pirate" causes the game to play a small bit clip of "Drunken Sailor" and show a picture of a Pirate, while a text box above reads "Your privileges to this game have been revoked because you are a pirate! 'Yo ho ho!'" See it here! In a straight example, before starting, the game would ask you for a certain word in the manual (for example, the fourth word in the second paragraph on page 3).
King's Quest V: Absence Makes The Heart Go Yonder!: Randomly during the game, you have to cast a spell from Crispin's (dead) wand to get past mundane parts. To cast the spell, you would have to look up the symbol on Page X of the manual. Entering the wrong code made the game unwinnable. It was excised in the CD version, which then created a problem in that you didn't remember you had the wand when you needed it at the end of the game because it literally had no other use.
King's Quest VI: Heir Today, Gone Tomorrow came with a "Guidebook to the Land of the Green Isles", which contained clues needed to ascend the Cliffs of Logic. A wrong answer would cause the stone step you stood on to retract, sending Alexander plummeting to his death. Later on the same island was a puzzle where you had to step on floor tiles in an order hinted at by a poem in the guidebook.
The first Laura Bow game (The Colonel's Bequest) required you to identify a fingerprint from a sheet that came with the game; originally, the fingerprints could only be viewed with a special magnifying glass, but this was too unfair (since some of them were fairly similar to begin with), so later printings just did it in black and white.
Its sequel, The Dagger of Amon Ra, also required the player to dig information out of the museum guide book that came with the game. However, since in this case the questions all revolved around real Egyptian mythology, they were the kind of thing an obsessive adventure game player would already know.
The first game originally didn't have copy protection, but age protection - to play the game, you had to answer a question that you'd have to be fairly old to know the answer to. (Presumably.) See for yourself. The VGA remake added actual copy protection questions based on the included Feelies.
In Leisure Suit Larry II, you have to insert the correct phone number of a woman by inputting it from the manual.
In Leisure Suit Larry III, there's a promotional code you have to type (which is in the a certain page of the magazine the game came with) when presenting your show ticket. Another which you have to know the locker combination.
In Leisure Suit Larry 5, to obtain airline tickets, the player must enter the corresponding symbols from the Aerodork timetable, which was printed in black on red in an attempt to make it uncopyable.
Police Quest (VGA): The combination to the main character's locker, which you needed to get into to retrieve his uniform, was the score of a football game reported on in the fake newspaper included with the game, and also inputting violation codes while putting an arrested man in jail. The sequel required the player to identify the last name of the person on a mugshot before playing the game.
Quest for Glory II had the map of Shapeir, though luckily you could guess your way to the money changer, and from that point onwards purchase an ingame map.
Quest for Glory IV: In order to get potions from Dr. Cranium, the player needed to help him remember the "formula" for various elements that went into the potions. Interestingly, the copy protection may not seem to matter since it's "just potions"; however, one of the puzzles required to beat the game requires a potion, meaning that without the manual you can effectively do everything except beat the game.
Robin Hood: Conquests of the Longbow featured a number of puzzles which involved having to consult the papers which came with the game. On the plus side, reading through these provided papers allowed you to learn about everything from medieval heraldry, to a secret "hand code", which used letters assigned to different parts of a hand to spell out words, to the purported magical properties of gemstones and trees. There were also dire consequences if you failed.
In Space Quest 1 (VGA), to get the cartridge, you had to enter the symbols from the manual corresponding to the term the dying scientist told you into the library computer. A second copy protection code was used for the coordinates of the Deltaur near the end of the game. Definitely copy protection overkill. The original EGA game didn't have ANY copy protection.
Space Quest 4 had its copy protection when you first enter the timepod, and you have to use the Space Piston Magazine included with the game to solve the code. The CD version excised it.
Space Quest 5 has the codes you need to enter to get to the various planets in the manual. Since you need to keep entering the codes throughout the game, it's borderline overkill. And there's no brute forcing here: going to the wrong coordinates wastes time getting there and kills you as soon as you're there.