Another possible problem is with the fit of the crank bolt in the spindle. I once had an old Campy BB spindle on which the drive side crank kept coming loose, no matter how much I tightened the crank bolt. I stopped immediately when I felt it, and I saw no evidence of damage to the crank. It turned out that the bolt hole in the spindle was loose, so the bolt just would not hold. Once I bought a new BB, I put the original cranks on that and bolted it up, with no loosening at all.