Dungeon Masters, which of your ""sure why nots"" had the biggest unintended repercussions?

Oh man, I remember this one campaign where I let a player keep a small, seemingly harmless jar of "pixie dust" they found in a low-level dungeon. I was just trying to be a cool DM and figured it would just add some flavor to their gear, maybe help with a charisma check or something. Well, turns out the player decided to mix it with some moonshine and tried to enchant a local stray cat. Long story short, that cat turned into a massive, invisible, and incredibly territorial predator that ended up stalking the party for half the campaign. It completely derailed my carefully planned political intrigue arc because the players were too busy trying to trap their own pet.

Another time, a buddy asked if he could have a "broken" sword that supposedly whispered secrets. I gave him the green light because I thought it would just be a cool narrative hook for finding hidden doors or loot. I didn't think about the fact that this guy loves to press every single button he sees. That sword ended up whispering instructions that led them straight into the vault of a high-level necromancer way before they were ready. The whole session turned into a desperate escape run through a collapsing tomb, and they ended up losing their main ship in the process. It was a total mess, but man, we still talk about that disaster at the table.