<Whoops, necro post. Had my search in a different window and got mixed up, sorry>
My PCs came into a room. It is clear that something is bad here, there are a handful of adventurer corpses. They appear to be covered in rust, as is much of the floor. There are three rust covered chests at the other end of the room.
Upon closer inspection, it is mold not rust. Here comes the fun bit, it takes a great perception and nature to realize that there are actually two types of mold here. One a bit more yellow, one a bit more brown.
The skeletons rise up, some are covered in yellow mold, some in brown, which really look quiet similar. Same with the floor. One mold can only be killed with fire, but you better watch your aim, if you hit the patches of the other sort, it will grow massively and suck all the heat out of the area, including you. As for fighting the skeletons, if you smash into them with your maul, you better be prepared for the huge cloud of spores you just released. Particularly if the mage was looking to use a fire spell anywhere near you.
Of course, all but one of the chests is trapped, but if they solve a riddle they heard earlier, they know that the gold one isn't. Pretty hard to tell which one is gold when they are all covered in mold. To make matter worse, the person that designed the room was a dwarf artisan (who had nothing to do with the mold, that moved in later), he would never mistake corroded brass or shiny copper for gold. The correct chest is white gold, better hope the PCs realise gold comes in different colours and know how to tell it apart from silver.