For me: walking in the Federal Plaza of Mexico City; seeing a young man walk deliberately up to a guard (at one of their many front facing doorways), pulling a bandana up to cover his face; watching (from 15/20 feet away) as he pulled a machine gun out from his jacket and cutting the guard in half; turning quickly away; lowering the bandana and the gun as he dissolved into the crowd. Beginning of the Chiapan Indian uprising.