People nod and wave at similarities. Two people passing on motorcycles do a low wave. Bikers in biker clothes not on their bikes nod. People passing on bicycles are 50-50. People with the same breed of dog tend to not do it. People with the same clothes on almost never do it. But people with dogs dressed in the same clothes absolutely will wave, and probably stop to let the dogs meet.
What do you do when someone knocks on the stall door in a public bathroom? I'm genuinely asking because I feel like every time I'm making it up on the spot. People say “occupied?” with a question mark because nobody wants to assert their presence. You don't even want to hear the sound of your voice in that situation. You're like "this is what I sound like when my bare butt cheeks are out”. I’ll say, "come back with a warrant". If that doesn't work I start crying and say "hold on I'm eating".