An off duty deputy sheriff who is a friend of mind came up behind me as I was leaving Sam's Club today. I hadn't seen him and he stuck his knuckle in my spine and started to say "SirI'mgonna..." By which time I'd whirled and swept his arm out of the way, and fortunately some part of my brain recognized him and hit the abort sequence on my reflexes. He laughed about getting my heart beating. I tried to politely point out that this wasn't that wise given recent events. When I didn't seem to get it I said he hadn't seen where my right hand was going by reflex.
There are times and places for jokes but remember that the butt of yours might be feeling a touch on the paranoid side that day.