What it says in the title; if you open a Crate's inventory and hold shift for a while, you will start crouching inside the inventory.
This is weird and inconsistent, and certainly a bit distracting when you want to shift+click a bunch of items, but seemingly not game-breaking.

I have yet to encounter this or any similar issue with other GUIs provided by the mod.

Actually, are the crates a Charm thing, and I've just conflated the two?