I was reminded of this possibility thanks to some annoying ants. We had some ants causing an issue where I live, so I put up an ant trap. Basically, poisoned bait.
The bait attracted ants and the poison killed them. But when it came time to freshen the trap, I found a spider had set up shop directly over and around the trap. The ants headed for the bait, but instead got caught by the spider. I just left this alone, figuring, well spider, you do you. Net effect is the same to me.
This makes for an interesting encounter as well. Predators, especially ambush predators, will lurk near a form of bait that will attract prey.
But that's not the say the actual bait must be pure, healthy, and rewarding. The bait itself can be another trap and the ambusher just piggybacking on it. This behavior can explain a lot of "monster with trapped chest" situations where the monster isn't the kind of being that would hoard and trap treasure.
Here are a few I can think of offhand for a typical fantasy game:
- "gold" coins that are actually cheaper coins dusted with yellow mold, paint plus contact poison, or which trigger a trap when approached (a pit, wall-shooting spears, whatever) plus an attack-from-above monster like a spider, obsidian jaguar, lurker above, quasi-intelligent ooze or slime, etc.
- a covered pit, with a mimic pretending to be a section of wall about 15' back, to better come out from behind delvers who stop 10' out when the lead character taps and hears a hollow sound back.
- an illusion of treasure set up over a trap (pit, slide, chute, deadfall trigger, etc.) with a charging ambush predator lurking down the hallway, hoping to ram someone into the trap and retrieve them later.
- an Avoid spell meant to discourage moving down a safe path and into a trapped area (or a dead end, or down the wrong maze path) combined with a Living Pit that sweeps under the delvers when they move into the area.
None of this is really new; nested encounters like this have been common since the modules made for 1st edition AD&D. But it's a good framing to keep in your mind - the monster might actually be piggybacking off of someone else's trap. They might inadvertently be saving you from the trap - a spider snagging giant rats on their way to poisoned water is keeping them from being poisoned, after all. And it'll help confound delvers who'll logically reason that Monster A couldn't set Trap B so Treasure C must be safe. It might not be - it's a nested encounter not originally planned for by the trap setter.