You ever try to take water from the dead sea home with you? My friend did. He put it in a jar and took it home with him. He put it on his shelf near a window as some sort of trophy. Anyways, the water was so salty it evaporated through the glass and all, real slow, but steadily. He didn't learn till later that you are supposed to keep it away from the sun. Anyways, as far as potions go. Their substance is partial physics, partial magic. What's to say the magic part can be contained by a physical cork. Or maybe the wizard making the magic messed up and didn't put the proper spell on the cork to contain the magic. There's a hundred different possibilities. Maybe you were running around in too much of a hurry, and accidently busted your potion open. When the game tells you, 'A potion evaporates from disuse', it's a roleplay way of saying 'Hey buddy, you can't carry a fragile glass bottle around with you forever without it breaking somehow.'