Probably the most expensive mistake I made was in buying a vehicle. It was an '80 IH Scout with a Nissan turbo diesel. This vehicle did great for a year and a half, but I had to take quite a bit of a loss. First, I didn't realize how bad the rust problem was when I bought it (probably the biggest mistake), and second, I hadn't realized until nearly a year later tha a previous owner had souped the engine up in an unsafe manner, leading me to do terminal damage to the engine (it was fixable, but the rust was so bad as to make it not worth it).
The reason I call it a mistake was because I paid $2000 for it. In retrospect, it was worth nowhere near that much due to the rust being so bad (one body mount was completely destroyed, making the body sag to one side in relation to the frame). It was probably worth the $1000 that I got when I sold it, when I bought it. However, live and learn...the next vehicle, I got very lucky in that the owner allowed me to make payments, so I had reliable transportation.
In spite of that, I had a lot of fun with that truck (played in the desert a few times), and don't regret the purchase...I just wish that the rust hadn't been so bad, or that I would have seen it and opted to hold out for another vehicle...