The first gruesome death comes from Ian Livingstone's Crypt of the Sorcerer.
You are mesmerised by the beauty of the sparkling diamonds, but picking one up is the last thing that you will ever do. Your body immediately crystallises and you sparkle as if you were a diamond yourself. You have saved Allansia from eternal doom, but you have paid for your greed with your life.
|He's not here for the fishing.|
Some books have other rules that are not moral but you need to follow them to survive. For example in Phantoms of Fear, by Robin Waterfield, you reach a point where you can enter the dream world and become invisible but you will not become immaterial. There is a point where you come across a bunch of lizard men who kill you. If you decide to become invisible, they still kill you because you are not immaterial as the book told you earlier.
|The phantoms often suffer from sinus problems.|
So if a gamebook (not necessarily a person in a gamebook as they may be a lying traitor) tells you something, it is probably good to follow that rule or you will end up dead.