My biggest financial mistake was co-signing on a car loan with a female friend-with-benefits. She had joined me in an MLM business (another large financial mistake on my part - I spent down a $15k inheritance from my grandmother trying to make it work) and needed a car.
She drove the vehicle and agreed to let me know if she ever had trouble with it financially. Our on-again off-again FWB relationship continued amicably, we even told each other about the new people we were dating.
I was speaking with a mutual friend one day who remarked, oh btw did you hear about so-and-so's car? The bank took it. The same day - I'm sure my poor emotional reaction contributed - I crashed my own car. Nobody was injured, but now I actually needed the repo'd car. I asked the bank how much they wanted before I could pick up the car, and they replied, we're not sure you're going to want the car given its condition.
My credit cards were almost maxed, then I borrowed to the limit to come up with the cash (~$950 if memory serves) to get the car back from the bank. The car was full of laundry, McDonald's bags, mostly-empty soda cans thrown in the back... and the glove compartment was loaded up with parking tickets and a few moving violations for expired tags. I called the jurisdiction in which all the tickets had been accumulated - I was also living there at the time - and they told me it was ~$2500 (this was the late 1980s), almost as much as the balance on the loan at the time. I was lucky that the bank and not this particular jurisdiction had picked up the car.
Since I didn't have the cash for the parking tickets, I called my father and had him register the car in another jurisdiction. Nobody ever followed up by matching the VIN, I guess computers were pretty dumb back then. Crisis narrowly averted.
I picked up a new fender from a junk yard to fix the one my ex-with-benefits had mashed in, fixed the broken window, changed the oil & filter, and asked her to sign over the title to me because she had broken our agreement. Amazingly, she signed! She was also repaying me for the three months of back payments, repo, and storage fees the bank had charged. Then she moved away and gave me a check in exchange for picking up her possessions. Of course, the check bounced. I spazzed and threatened to file bad-check charges, a felony. A money order showed up in due course for the remainder she owed me.
Fortunately I made these mistakes when I was young & foolish and had little to lose. I resolved never to cosign on a loan with anyone with whom I wasn't married, and never to join another MLM. Probably not a huge mistake all things concerned, though I was this close to bankruptcy.
I found ever better-paying jobs, controlled expenses, and paid down the CCs so that I could settle up every month without incurring interest. Lessons learned and passed down to the next generation ("Dad, you were soooooooooo foolish!!! You cosigned a car loan for sex??").
