It's called critical incident amnesia. Memory loss is common during extreme high stress scenarios such as discharging a weapon.
http://dailycaller.com/2013/12/12/why-you-can-win-the-gunfight-and-lose-everything-else/
http://dailycaller.com/2013/12/12/why-you-can-win-the-gunfight-and-lose-everything-else/