I had this happen on a shakedown ride after I resurrected a '74 Motobecane Grand Touring. It was pretty defeating when I noticed that the crank was also slightly damaged. I limped it home, and found the spindle threads were a bit stripped. I broke out a tap and die set and rethreaded them and fit a matching nut. It still loosened a bit on the next ride, but loctite and a thin lock washer solved the problem entirely.