Well, I was gonna say saddles, but as I type this I'm thinking that even a poor saddle choice isn't a complete waste as it still serves as an example of what doesn't work. I have a clear understanding of the size, shape, level of padding, etc, only because of my earlier investments in poor choices.
I do, however, regret all of the time, energy and money I spent on lighting solutions before finally embracing dynamos. I have an embarrassingly large collection of battery powered headlights and taillights and an even more embarrassing legacy of dead batteries which all could have been avoided had I made the move to dynamo lighting much sooner. First it was alkalines, then rechargeables, then USBs. Batteries die. They become unrechargeable. Sooner or later it happens to every battery ever made, so if I'd have figured it out sooner... oh well.
-Kedosto