My favorite was this 82 year old man in Independence, MO. I went on a tune-up and he had this 40 year old equipment that he wanted us maintain, even though he already maintained it himself.

He was in great health for an 82 year old man and wanted me to know about it. 

He asked me, "Have you ever seen an 82 year old man do this." 
 
He started running up and down his stairs from the main floor to the basement, around 10 times. Up and down, as fast as he could. He then came to me, as serious as he could be, and right in my face, said he could probably beat me up if we got in a fight. He started doing karate on me. Coming within a few inches to my face and my body as he did high kicks and karate chops. He was dead serious about competing with me. I had no idea how to react, so I would just smile and laugh and say, "yeah you're really healthy, I agree you could probably take me down."

I continued going to his house year after year and he would always make me laugh by the way he acted: showing me how much he could lift, how high he could jump, even one time going outside to watch him run up and down the street. His wife would always sit back and laugh at him as well.

I really don't think he even cared about me maintaining his equipment, he just wanted to brag how awesome he was to me.