Is if there were no echo when your potion "went bad", and only people specially trained on potions (such as alchemists?) could tell if a potion was still good or not. If you quaffed a potion after it had lost its potency, depending on the potion, it could be like drinking water, just getting a lower-duration/strength of the original potion, or it could be much worse. What if a degraded potion was actually poisonous? Or the polarity of the magic flipped (invis becomes faerie fire)? Even if the potion just turned to water, you would be stuck not knowing that the potion wasn't going to work... and you would have been lugging around dead weight.

But how much fun would that be?