I purchased a 2015 GT 50th anniversary. I have owned it for 3 weeks - no issues with fit and finish, so I wouldn't be condemning Ford for a 1 off. Could have happened in transport.
2015 Mustang GT 50 Year Anniversary, Ruby Red coupe. Staggered fit 19 inch wheels from American Muscle. Front tires are Sumitomo 245/45 19, rears are Sumitomo 275/40 19. Sound tube deleted, resonator deleted, replaced with h-pipe, Bama tune, custom sub woofer with Rockford Fosgate P3D4 10 inch subwoofer driven by and Alpine MRV 500M mono power amp. Replaced Sync 2 with Sync 3.