When I try to fill a bucket from a stack, and there's no free space in the inventory, the player throws the filled bucket away rather than just not being able to fill it in the first place. This might be intentional, but I suspect it's a side effect of how filling buckets is coded - it removes your empty from the inventory and treats the full one as an item drop. Either way, I hate it. I keep losing buckets to lava pools this way. It would make more sense to run a check against the player's inventory first and returning an invalid action if there's no free space.