Just rearranging some stuff, I came across an old travel litter box, from the days when the kittens were small enough to pop all three of them in a small dog carrier and take them to the office, and I had an office to take them to. It was actually a plastic file box, sized to hold maybe a ream of legal-size paper; with the lid on, it wasn't exactly airtight, but good enough to prevent litter spills while in transit. With the lid off, it was big enough for the use of a small cat.
Well, it's not much use these days, so I figured I'd dump the new-old-stock litter into the current main box.
Huh. This litter looks funny. I'd used a couple of light-tan varieties, before settling on Precious Cat. This stuff was granular, as I remembered, but... sticking together... oddly. And I was pretty sure I'd put it away with unused litter in it. Clumping from past humidity, maybe?
When I opened it, I had the answer. Moths! Pantry moths had set up housekeeping in the kitty litter!
I guess it must have been one of those brands made from vegetable detritus, like wheat hulls or something. And, somehow, it's nutritious for moth larvae.