Expired Goods
Magic items are, by definition, magical, and as such are preternaturally durable. It wouldn’t at all do to discover a wondrous magical sword jutting from a medium-sized boulder in a courtyard that was rusted beyond use after a couple decades of exposure to the elements. Potions stretch this idea somewhat, as they’re a substance you put inside your body. Even straight water in a flash gets kinda icky after a few weeks, let alone centuries in a musty dungeon. Even if the potion itself didn’t degrade or go off, there would be ample time for bacteria to grow and fester inside the container. Drinking a potion should include a not-insignificant chance of horrible food poisoning.
I read the mouseover and plugged the given dice into Anydice.
With an ‘expired potion’ that heals 6d4+3 hit points but deals 3d8+3 poison damage, it’s possible for you to take up to 18 points of damage or heal up to 21 points of HP.
98% of the time, though, your HP will increase/decrease by +12 to -9.
On average, you are very slightly more likely to be healed than harmed, though only by a couple of points.
Of course, if you have poison resistance, it’s a different story.
A poison resistant or immune race might create these ‘expired potions’ on purpose, to stymie adventurers or other enemies who might steal and use these potions against them.