I don't know how common these things are in the boxes we find, but I came across this:

a knitted black wool cap finished with a scarlet puff

I had my young bard do his best to sing to it, found out that it holds the Ranger spell, Resist Elements. Holds several charges, crumbles after last charge.

I'm not sure if there's more to it, but I thought it was kind of entertaining to have to drop 4k on a box from a shelfae to get the town locksmith to open it and all I find are a couple of cheap gems, 1.5k in coins and black wool cap.

-Drumpel

That would be an item from the treasure hopper. They always cause the NPC Locksmith prices to go into overdrive.

GMs sometime design items and drop them into a system that will randomly select a critter-dropped box to feed one of the items into. They can be clothing, weapons, armour, magic items or other functional or fluff -- anything goes, really. You can usually tell a GM-designed hopper item from a randomly generated item by the description -- long descriptions or detailed shows tend to mean it was designed rather than generated.

That's a neat one, but I'm surprised it's crumbly. Usually they won't go to the trouble of designing something only for it to destroy itself.

Some awesome items get hoppered, and some really strange ones do too. My old character in Plat once found a pair of canvas trousers that had in their pockets: a small pebble, a clear tourmaline and a blood-stained handkerchief. He also once found a vomit-themed gown.

Dave, Brandain's Bard

You can buy that sort of clothing in one of the Icemule shops. I am not sure if that exact description is in the shop, but there are very similar items.