I actually noticed this when I first signed up for Beta 3. But seeing as it was a bug that I had encountered within my first few minutes of playing, I thought someone else probably would've taken notice of it already, and I subsequently put it out of my mind. But this issue is still present in Beta 3F.

In order to invoke the bug, the proceedure is thus:

Purchase 4 of any single usable item (Scrolls, Potions, etc.). All four items must be of the same type, so that they will stack together. As you purchase them, three of these items will occupy the F1 slot, and the fourth copy will occupy the F2 slot.

Now, try to click on the F2 slot to use/consume the fourth item. Instead of using up the item in the F2 slot, the game will consume one of the items in the F1 Slot. So you'll actually end up having 2 potions/scrolls left in F1, and the "fourth" potion/scroll will remain untouched in the F2 slot. "Dammit!" You will say. "I wanted to free up my F2 slot so I could purchase and place another item in my limited array of item slots!"

Incidentally, the items in F1 and F2 will be stacked together into a single slot again after you die & respawn. But until then, you're stuck with two item slots being used, when one would (or should) suffice.