There is an old story about Morihei Ueshiba and his Daito-Ryu instructor. One day, they were playing a game of Go. And one of Morihei's martial arts friends came in the door. As he had his face covered by a scarf due to the bad weather, Morihei's Daito-Ryu instructor could not recognize him. He immediately picked up the Go board and bashed the guy on the head with it several times. When it was discovered who the person was, he sheepishly said "I thought he was an enemy in disguise coming to kill me".
This story can be found in the book Invincible Warrior about the life and times of Morihei Ueshiba.