We've all had (at least) one. Here's mine:
1966 Mk1a Sunbeam Tiger. Bought it from a friend who'd lost interest in the build. Body work was done, paint was done. Engine ran great. Had a new clutch, new brakes, rebuilt driveshaft. Had almost all of the parts, a new interior, a new soft top, a very nice hardtop. All I really had to do was chase down some electrical gremlins, source some rarer trim bits, have the bumpers plated, refresh the suspension, and reassemble it. Seemed simple enough, right?
While I was doing the suspension, I decided I wanted to have the front crossmember reinforced. The plans for doing so were readily available, but I didn't have the tools to do it myself. I located a shop that specialized in old British cars and learned that the owner was a Sunbeam guy and ex-dealership wrench. He knew the job and had done several. He also knew the car, having done some some work on it for the PO. The deal was struck and since it was drivable and the weather was nice, I drove it down to him. All seemed well...
One evening I get a call from him and he's obviously upset. His place was broken into and my car was stolen. That was bad enough.
Then, about a week later, I get another call, this time from the police. They had found my car. The thieves had hotwired it (badly - they drove it with the bendix engaged), drove it all over the back roads by his place, ran it out of gas, kicked in every panel on the car, set it on fire, and pushed it into a ditch. Then I got it back.
I got it running again and sold it for a loss. The guy that bought it turned it into a drag car. 10 years later they were selling for nearly 10 times what I had in it. Sigh...
What's your heartbreak project?