Your party walks into an alchemist's shop or what have you, and there's a bargain bin full of unlabeled bottles. A dm has many many options for what these could be, but for some trollish ideas, consider the following.
abracadadderall: drink this potion and you become able to maintain two concentration spells at once, but you develop insomnia and can only take short rests for up to 1d4 days.
potion of become potion: what it says on the tin. you become potion. the next person who drinks the potion frees you, but then becomes the potion themself.
suspension of disbelief: you believe everything you are told for up to 1d4 days
water walker's potion: manifests water under your every step, making every patch of ground you walk on unpleasantly wet. does not let you walk on water.
essence of cropdust: you let one rip uncontrollably once per turn for 2d12 turns. negates stealth and gives enemies advantage on attack rolls.
potion of unlocking: when drank, it gives you a key. you have no idea what it unlocks and you'll spend the rest of your campaign searching. the dm can decide if it actually unlocks anything at all.
elixir of rixile: forces you to speak backwards for 1d4 days