I regularly use a metered dry powder inhalation dispenser [link] to control asthma. To use the device, one pulls on a thumb-operated lever, and a spring-loaded trigger punches out a precision dose of powder, which is then inhaled.
The device is pretty clever, and displays an index of remaining doses
which counts down to zero as the medication is dispensed.
One difficulty occurs around the last dose. Since I frequently use the dispenser without benefit of eyeglasses, it can be difficult to know when I've used the last dose. The powder is fairly tasteless, and the mechanism operates the same whether the index has reached "0" or not.
So I propose that this device should either fail to operate when empty (probably best,) or at least give some indication that one has attempted to go below zero (-1, red block, "EMPTY" or some such.)