In 1992 2 C-141’s collided during a formation air refueling. We had detailed procedures to insure separation between multiple receivers and refuelers. They worked fine for decades. But on that particular evening, one C-141 deviated from those procedures and descended quickly while simultaneously turning towards his post refueling position, a big no-no…the procedure was to descend to your post refueling altitude, THEN once at your altitude turn to attain your lateral position. That crew didn’t follow the proper sequence and collided with another C-141. It doesn’t take much.