An old trick

I was thumbing through my old copy of B2, and discovered a couple sheets of paper folded between the pages, which turned out to be the slightly faded map and dungeon key I had made for the Cave of the Unknown. 

Included was this dastardly trick, in location #5.

A wooden chest sits by the east wall.  It is trapped with a capsule of gas (actually harmless fog.)  Inside are 3 copper coins and a note that reads, "Was it really worth it?"

I must have been about 14 when I wrote that.  Good times.