My worst wasn't a huge loss. My brother and I built a dirt bike from a pile of spare parts: 250 frame, with a 175 2-stroke motor chicken wired on, and a gas tank off a 70cc kids bike. We had some fun in the back field, as long as we kept the chicken wire twisted tight. We let this townie guy ride it, who said he had experience riding dirt bikes. Well, he didn't know how to shift, and took the bike for a 2 mile ride in first gear. After about 5 minutes, it welded itself together, and all the parts went back into the pile. Easy come, easy go.
Two of my good friends had a worse experience. One guy let the other ride his pristine Yamaha Seca II, and he low sided it on Highway 9. The friend stepped up and bought the bike for what is was worth before the damage. He fixed it up and rode it for years. The money that he paid went toward buying a CBR600F1, so the first friend was quite happy about the whole thing.