One of my friends and I have been pulling a practical joke on another for just about as long as we've known him where we come up behind him and grab his shoulder. So I go to a big high school, with lots of people. So I come up behind this guy who I think is my friend and quickly grab his shoulder and whisper " I watched you sleep last night." Turns out it wasn't my friend and I got the most confused and awkward stare I have ever seen. I didn't try and grab anyone's shoulder for a couple of weeks after that.