You can overcure, all that'll happen is that the meat will take on an almost pearlescent hue to it as opposed to the deep pinky red that we like.
It's also called "Cure Burn" nothing wrong with the meat, it just doesn't look as good....and yes, I know this 'cos I have done it....once!!!!
(left a small pork loin in a dry cure for a month to see what would happen!)
Pro Q Excel 20 - Brinkmann Pro Grill - ET-732 - Various Cold Smoke Generators - Smoking Gun