Have an item equipped and another of the same item (possibly with different enchantment) in the locker.  Tap the equipped item.  It'll have a "Remove" button.  Tap that button.  Crash.  Consistently reproducible.

I originally discovered this with a CD's Griffin Slayer.  I had one from a locker on level 2, and found another in a shop later on, so I stashed the second one in the locker on the dungeon boss level.  I then tapped my equipped Griffin Slayer to make sure that I hadn't stashed the one I'd mastered, and at that point I noticed the "Remove" button.  Naturally I had to try it. :)

You might not have to have the duplicate item equipped; I haven't had the chance to check.

rdermyer said:

You do not have to have the item equipped. I crashed the game with trying to put arrows into the locker when I had arrows in it already. Or maybe it was when I tried to remove the arrows when I already had arrows in my inventory.

That sounds like a slightly different issue; I haven't tried putting stackable items into a locker when a stack already exists in the locker, or taking a stack out with one already in my inventory.  The bug I described occurs if you have an item in your inventory and the same item in the locker; the item in the inventory has a "Remove" button that crashes the game.

Sounds like the locker might have more than one bug involving duplicate items.