In my Eberron game, one of the characters is a Tabaxi Arcane Trickster with a background with House Cannith who's very into magic item crafting, so the player is always on the lookout for rare and unusual components. They were making a foray into the Mournland, and throughout their journey I really played up the concept of the place corrupting living beings and bringing magical effects to life. Every time they cast a spell they had to make a Wild Magic style check to see if it would turn into a living spell, and one early encounter had them attacked by some magic items that had become animated.
Their journey took them to the Glowing Chasm, and as they gradually came closer I played up the fact that the corrupting effects were increasing the further they went. They faced several examples of corrupted wildlife, and each morning they had to roll a d20. When one of them, the Tabaxi, rolled a 1, she found that her character had undergone a subtle mutation, becoming more feral - she no longer enjoyed cooked meat, and had an urge to hunt down any small animal she saw.
So when they got to the edge of the chasm and saw that it was lined with glowing purple dragonshards like the inside of a geode, and teeming with large, mutated insectoids that had been corrupted by these crystals, I felt like I'd made it clear that this was not a place to be messed with. But I really, really should have known better.
The player of the Tabaxi immediately declared that, before they left, she had to get some samples of these dragonshards. She understood that their energies might be dangerous, so she emptied out the party's Bag of Holding, rappelled stealthily a little way down the cliff edge, and harvested some shards, putting them in the bag.
At that point I ran an unplanned encounter in which the nearest of the monstrous forms in the chasm - a Purple Worm - noticed her, and attempted to eat the party shortly afterwards. They got through that experience with the party's cleric unconscious inside the worm's gullet by the end of the encounter.
But the true sting in the tale happened three sessions later, when they visited a contact they'd done business with in Arcanix, hoping to get their dragonshards assessed and evaluated. In all this time they hadn't accessed the Bag of Holding, opening it only once they were in this professor's laboratory. That's when they found that the dragonshards had integrated themselves into the Bag's interior and turned it into a living item, which they discovered when it promptly swallowed the Tabaxi when she opened it, along with most of the lab's equipment and an apprentice. The ensuing battle ended with the bag thoroughly slain, but the apprentice and lab equipment, which were still inside it at the time, were ejected into the Astral Plane. The Tabaxi only made it out of the vortex because she was wearing Winged Boots and was able to fly out before it closed.
The players were upset mostly by the loss of the bag, and the fact that the professor wanted them to take responsibility for misplacing his apprentice.
