Fortunately I've never been so clueless with money to get myself into any real trouble, like credit card debt or financing a new car, but two mistakes stand out:
1. Since I was in HS, I wanted an electric car, so I decided to build one. I spent about a month and $9500 (almost all the money to my name at the time) building it, then ran out of interest and decided to sell it off. I sold the half-converted car for $1250.
2. As you can imagine, it took a while to sell the half-converted electric vehicle, even with 95% of the parts included. So while it sat in my mother's garage, I continued to drive my old inherited Corolla that was in good condition and consistently got 33-35 MPG, no matter how much I abused it (51 when I really pushed it). Being a car guy, I hated that car, and secretly wished someone would rear end me so I could collect the insurance money and replace it with something more fun. As it turned out, an errant shift to 2nd gear at 85 MPH did the trick, necessitating a $2200 engine replacement. I elected to scrap the car and borrowed $1k from my mom (I had spent almost all my money on the electric car, remember?) to buy a 21-year-old, 19 MPG Volvo 940. While I liked that car, I only had it for one summer because it was nothing but constant problems-- I was at the pick-a-part yard almost every weekend getting parts to fix it. Sold it for $400 when the cat went out.
So to replace it, I bought a $1600 Buick Park Avenue Ultra. This was the first car I owned I freaking adored. I could just stare at that thing for hours. Unfortunately, it was shortly after this time I started reading MMM, and began to see the folly of owning such an impractical, luxurious, 240 HP monster that ate premium fuel at a rate of 16 MPG (with mild hypermiling!). So I sadly traded it in for a $2500, 202k mile Scion xB, which is my current car. Mild hypermiling gets me 36-37 MPG, and normal driving yields 30-32, which is plenty adequate.
But between these two mistakes, I estimate I'm out about $16k. Not nearly as bad as some of the stories here, but I could be so much better off right now if only I'd never started the EV conversion and never destroyed the Corolla. Live and learn I suppose.