I have an irrational fear that someone is going to do this to me whenever I have to park the cart to go use the bathroom - what if one small object is removed while I'm peeing, and I don't even notice until I get all the way home?!
The total opposite of this happened once, and it was genuinely one of the weirdest experiences of my life. I was in the grocery store looking at packaged meats, and another lady comes up and parks her cart next to mine to look at them too. She starts making some choices, loading her arms up, but I'm not really paying attention because I'm comparing prices and thinking over my own selections... Until suddenly she starts putting the meat that she has chosen on top of my groceries, in my cart - that looked nothing like her empty cart, which she then grabbed the handle on and pushed away while I stood there watching, absolutely bewildered by what had just happened.
I did not buy the meats she had chosen for me, and I live in both fear and hope that our paths might someday cross again.