For a brief period during April Fool's Day, a wormhole opened between the universes of Godville and Star Trek. A few Tribbles slipped through, and they've been making a home in loot bags, Mystery Boxes, and the like ever since.
Tribbles are small, furry creatures that reproduce at an incredible rate; they are born pregnant. Although adorable, their ability to multiply (and the amount of food they eat) makes them indirectly very dangerous.
Tribbles will multiply to fill up more and more slots of a loot bag, preventing a hero from carrying more valuable treasures. Because few traders want Tribbles, the hero will have difficulty selling them, ending up stuck with an increasingly full loot bag for some time. Eventually (usually after a day or two?) the hero will find somewhere to sell the Tribbles, but it is rarely worth the loss of inventory space.