Bike related? Buying a custom bike that didn't work well for its intended purpose. My remorse was I could have gotten something along the lines of a Surly (I love my Steamroller and like it more than my custom bike) and have something that functionally worked better for a fraction of the cost, just not the bling bling of the custom. Well, not 100% true. Fit wise the custom is really good since it was custom fit, and now I can use that data to setup my other bikes.
Non bike-related was buying a classic Mustang from a friend's brother. Car wasn't as advertised and, long story short, I wound up losing thousands of $$$ on the deal. Its still affecting me many months later. If one wants to talk about learning the hard way. The custom and Mustang both occurred within the same year, so my losses this past year are huge (percentage wise compared to my savings & salary).
My lessons?
1) I loved the bikes I had, and I *knew* I could have been happy with a cheaper frame/fork, but more than anything wanted the bling of the custom (custom look, not just geometry. So handcut lugs and the like). I should have gone with my gut feeling.
2) Don't do business with friends/family.
3) If you ignore (2), get a written, signed contract if you're going to have them do lots of work worth lots of $$$ on a project, whether it be a muscle car, house, or whatever it may be.
4) Another positive from these two ordeals ... i decided to put together a real budget. This has helped immensely, and I look forward to not making these mistakes again.
OK, that's all my brain will conjure up at the moment