The error is that the chest itself should be trapped.
There is no mechanic in the NA rules requiring the hero to stand on the square adjacent to a Treasure Chest to open it, hence putting a trap on that square is pointless. The Hero will step into the room, search for treasure and get the contents safely, then leave without even having to search for traps! This error was introduced in the North American version, it doesn't exist in the EU versions of the same quest as I explain
here. It's a recurring issue because the map makers seem to forget this rule on occasion, when they clearly intended to make the chest itself a trap, plus they left the chest unmarked while the original version had it contain gold.
Had they gone back and compared the EU edition they would have seen it, but instead they just copied the old error.