There's a trick I like to pull on my kidlets whenever we're in a public bathroom. I'll be in one stall and a kid-o-mine will be in the stall next to me and I like to reach under the wall and grab their ankles. I've done this to them for years.
One weekend, big kidlet and I went with relatives the the Arboretum so I could take some portraits. As usual, kidlet and I wound up in the bathroom at the same time. As I was sitting there minding my business, I looked under the stall wall and saw a bright red pair of tennies. I thought to myself 'oh goodie, I'm going to yank her ankle.' I did the ankle yank and her foot moved but she didn't say anything.
As I exited my stall I noticed that my girl was waiting for me by the sinks! And she was wearing green tennies, not her red ones. Oh holy crapezoid! I just ankle yanked a stranger! Poor girl, she was probably terrified to come out of her stall! She finally did and I apologised profusely and we all had a good chuckle over it.
Later, I noticed the girl telling the story to her family, some of which had just asked me to snap their picture. So she and I told the story of what happened and we all laughed at my idiocy.
The bathroom ankle yank is a good trick to pull - people don't usually expect it (with the exception of my girls). But to save yourself some embarrassment, make sure that it's someone you know next to you.