2010 Springfield Armory M1A Standard
I was going to build a precision rifle from an M1A Standard. I still love the M1A, but unfortunately the one I ended up with was a complete dog. It's a two year long saga, but the short version is: I had over 3k wrapped up in the project already (Rifle, custom stock, etc). When it came time to add glass, I learned just how wildly out of spec my receiver was (and over two years learned a lot about the challenge and expense of accurizing M1As). The rifle functioned reliably, just wasn't going to allow for a scope mount. I never did get to shoot the thing in the custom stock I ordered for it, so maybe it would have done better, but out of the box accuracy seemed to be poor (6" @ 100 was the best anyone ever got it to do with FGMM and iron sights). With those two issues, and the knowledge that a SCAR H would suit my desires equally well (and more cheaply and and with less headache), I decided to walk away from the whole project.
As another poster said, it's not so much a regret because I learned something along the way. But... considering how much time and energy went into that project, I do which I could get the time back.