Officers are trained to take a gun from an assailant in close quarters, a maneuver they practice by role-playing with a fellow officer. It requires speed and deftness: striking an assailant's wrist with one hand to break his grip while simultaneously wrestling the gun free with the other. It's a move that officers have been in the habit of honing through repetition, taking gun, handing it back, and taking it again.
Until one of their officers, on a call in the field, took the gun from an assailant and handed it right back again. In a mutual astonishment the officer managed to re-seize the gun and hang onto it. The training regime had violated the cardinal rule that you should practice like you play, because you will play like you practice.
Peter C. Brown and Henry L. Roediger III, Make It Stick: The Science of Successful Learning