Oh, mankind, will you ever cease to surprise me with your behavior? Probably not...
This morning I was in the cafeteria getting coffee. You fill your cup, pay at a register, then go over to a sort of circular island where the cream and sugar live. It has a two-tier surface, with the aluminum containers of milk and cream on top, and a lower level where you can place your cup while you add your preferred fixings to your coffee.
Like most people, I add some cream to the coffee and then taste it to see how close I've gotten to getting it right, and then I usually have to add a little more, because adding too much milk or cream ruins it. (I have done this once or twice, and have had to go back into the serving area to add more coffee to my cup; the staff don't seem to mind, but it's a pain.) There's a period of a few seconds where I'm tasting the coffee, and I put the cream container next to me on the lower shelf of the island. After I'm done, I put it back on the upper shelf where others can reach it more easily.
So this morning as I'm doing this, an arm reaches across all the way from the opposite side of the island for the cream. The Unspoken But Agreed-Upon Etiquette of Self-Serve Coffee is that you wait for a person to be finished with whatever they are using before taking it to use it yourself (or, you know, ask if the other person is finished), but apparently this person was visiting from another planet where such behavior is not the case. I put my hand on the container and looked up at the guy and said, "I'm not finished with this yet."
When I was finished, I made a point of putting it back in the middle of the upper shelf instead of handing it to him, so he could understand how things are done in places where people have learned how to be patient and wait their turn.