Impulse buying an extremely low mileage example of a 13 year old car, simply out of nostalgia for my very first car. This car caused me to not have the money available to buy my actual and very rare (realistic) dream car. I ended up putting 5k miles on it over 2 years and selling it for exactly what I paid, but I missed the opportunity to buy the one car I really wanted. I settled on another one with the wrong options, the wrong color, and in slightly less good shape.