Forum discussions about the pros and cons of magic shops have me thinking about adventurers stocking up on scrolls and potions, since those are pretty much the only kinds of magic items I'd have for sale in my campaign. And thinking about a pack full of potions has me thinking of the downside to that: potions breaking. Everyone's heard of the Potion Miscibility Table and its various replacements, but I haven't heard anyone suggest that even a single potion breaking unexpectedly ought to have side effects.
For example, if a healing potion breaks in a backpack or sack, maybe there's a chance that it "heals" the leather or plant fiber, causing a cloth sack to sprout and a backpack to bleed; extra healing makes the container attack for one or two rounds. A broken potion of speed causes the pack to age and rot. A broken potion of levitation makes your pack lighter for a short period of time. A potion of invisibility makes the pack invisible, of course. A potion of invulnerability seals the container.
Many of the other potion effects could affect anyone nearby who inhales the vapors. Have everyone potentially affected roll a d6; divide each die result by 3, dropping fractions, to get the number of rounds the effect lasts. So, one third of the time, there's no effect; the rest of the time, the effect lasts one round or rarely two rounds. I'd probably use the same kind of roll for broken potions that affect the container instead of those breathing the vapors; it's just a matter of deciding which potions affect containers, and which affect people nearby.
The best way to decide which is which, of course, is to pick the result that would be more annoying.