Back in the middle 80s I ran a AD&D 1st campaign for Dwayne and Tim . It was the first evil campaign I ran where Dwayne played Lord Divolic a Myrmidon of Set (LE Paladin) and Tim played Count Travlin his ally. At one point in the campaign I just didn't have anything ready. So I pulled out the Tomb of Horrors.
When they saw this they warned me that they knew it by heart. Apparently when they first learned D&D all they had was the Tomb of Horrors and Keep on the Borderlands. They puzzled out a system using d6s from the few notes and stats and played them constantly. One would run the Keep while the other the Tomb.
Needless to days by the time they got the rules a few weeks later they knew them both by heart. Now a couple of years later I am about to run the Tomb for them. I really didn't have anything else so I said what the heck what the worse that can happen? I wasn't going to abandon a evening of gaming.
Little did I know....
The adventure wasn't hard but it wasn't a cakewalk. They realize shortly in that while very familiar with the place they didn't have every detail down. So they went about methodically and carefully. Now I know some of you referees are talking "out of game knowledge". However I agreed not to penalize them because of the circumstances.
So after a couple of hours they were standing outside of the stairs that lead to the demi-lich's resting place. The Tomb of Horrors comes with great "Show the Players" illustration along with short but evocative descriptions to read. So I read the description which included
The mithril doors set on the southern end of room 28 are 14-
feet wide and 28-feet tall (and 3-feet thick).
Dwayne suddenly perks up and looks real interested and says.
Rob Read that Again!
I did and then it hit me.
Oh no! Forget the demi-lich's treasure. We are talking the motherlode of Mithril here. What even more amazing that Tim and Dwayne missed that all the time they were running it themselves.
Well they dealt with the demi-lich by knocking it in a Bag of Holding. (What they did that is another story). Left the dungeon and made a deal with a couple of dwarves from Thunderhold. The wealth realized from the Mithril and the other precious metal lining the tomb fueled Divolic's subsequent rise to power.
While that much wealth was a potential campaign wrecker I turned it to a advantage not by figure out how to take it from the players. But rather using it to take the campaign to a whole different level.