Long time ago now, but mine was in my early twenties. Managed to buy two very unsuitable horses in succession over a short period of time 
They must have cost me thousands, to deal with the subsequent mess. Which was a lot of money going back almost a couple of decades.
I keep wondering if I'd put that money in a pension, what would it be worth now? I would have been off to a great start pension wise at an early age.
What stupid financial decision did you make, that sticks with you years later?