I joined a group in college playing a homebrew campaign that'd been running for several years already and was only in the first of what was planned to be three parts. Sometime prior to my joining, the party'd aquired a large sack of various potions of various (individual) colors. However, since nobody had Brew Potion or Craft/Knowledge (alchemy), they'd only been incrementally testing them. At the time of shennanigans, the only one's we'd determined for certain was that green potions were Cure Wounds and brown potions were Enlarge Person.
At the current time we'd found ourselves trapped in (what was essentially) the basement social hall of a church, about to be attacked by a group of red vampire knights. Having no other good options, one of our party (don't remember who, it was years ago) decided to just start chucking potions at them to see what happened. First he threw a white one and suddenly the entire hall's covered in 6" of fresh snow. Then he threw the one big black potion (which we'd all been very careful to avoid) and it created a large-sized Umber Hulk. Then without looking, he threw an additional brown potion onto it, which grew it to huge-size. The Ranger, about to have a fit, sent out his (medium-sized) bear animal companion and dumped a brown and a green potion onto it…while the original potion-chucker poured an additional brown one on it. The DM ruled that the three potions magnified and made the bear larger than the Umber Hulk…and it hit and broke all the remaining potion in the bag as it grew.
Thankfully, our Rogue had finally gotten the back exit open, so we booked it outta there and left the bear to what we assumed would be something rediculous. By the time we finished rolling down the hillside, we watched the entire church get ripped apart by the growth spurt of a more-than-collosal-sized bear glowing with an aura of rainbow energy. The Umber Hulk, not even coming up to the bear's knee, gets eaten in one bite and there's a flash of white light.
When we can see again, the bear is right back next to its master…but now with permanent large-size and improved stats. He was forever dubbed the Giant Elemental Bear of Death.