With Kleptomancy you can stack potions that you get from the passive with potions you buy from store
So waaaayyy back in beta, you could stack potions for extra healing over time. I.e if one potion is 10 hp per second, you could pop 2 of them and get 20 hp per second. Understandably, this was nerfed out pretty fast and now potions could only go one at a time (one hp pot or one mana pot etc.)
However now, with the Kleptomancy passive, the potions/biscuits you gain can be stacked on top of potions you have purchased from the store. This can grant you a ton of hp/regen early in the game (lvl 4 with 25 hp/second in some scenarios). Given that riot initially nerfed this potion stacking out early in League's career, I think this should also be looked at and probably nerfed out, basically just shouldnt be able to stack potions for tons of hp/regen
Screenshot: potion from store stacked with potion from Kleptomancy passive, 21 hp/sec regen at lvl 2.