It's easy to miss something while uploading/downloading a tune. So many of the individual systems in these trucks are monitored by the computer that you have to make sure the doors are shut and everything is either off or its fuse pulled to prevent a semi-automated activation (like OnStar). I screwed up a load one time that required a visit to the dealership to have them reflash the ECM. They know me quite well, and just smiled when I asked them to do a reflash...