Best way to describe this is through example: So I have two stacks of, let's say Spinmetal. One is maxed out. The other has five. I go to deposit the five in my vault, but something unexpected happens. Instead of dropping the five as I intended, it instead subtracts five from the maxed out stack and puts it in the vault. So now I have a stack of 195 and a stack of 5. It's kinda unfortunate.