I've had a few builds go south on me. I did an F-18 with Blue Angels paint scheme not long ago. Had a lot of problems with the landing gear on that one, but otherwise, it turned out OK.
There are 2 that really stick out for just horrible builds. The first was a Saturn V rocket. Assembly wasn't bad, but you would be surprised at how many problems I ran into when painting the stupid thing. I finally finished it, but the paint job was terrible. The other build was an F-15 I started a couple years ago. It was just an all around terrible build. Pieces wouldn't stay together, and no matter how much I tried to get the fit right, I had gaps you could drive a semi through, the list goes on. I put that one on the sidelines and haven't touched it since.