In the last year or two I've noticed a new habit of grocery shoppers: the parking of the cart, while they wander off down the aisle to look for something. Occasionally they'll try to leave their cart in a position that's not inconvenient for others, but come on, we're talking shopping carts here; there's really no way to do this without being a pain-in-the-ass.
So, I'm mulling over adopting the following strategy: Whenever I encounter one of these "parked carts" whose shopper is more than ten feet away, I would remove one item from their cart and deposit it later someplace else. Of course, this won't result in any kind of "lesson" being learned, but it would at least pose an inconvenience for people who evidently believe their carts become non-corporeal objects when they release the handlebar and turn away from it.
(Or is this too evil?)
UPDATE: In comments, Sean points out that sometimes parents get overwhelmed by their kids, thus necessitating questionable cart behavior. I can understand that, to a point, but I haven't seen too much of that, believe it or not. The main grocery store we frequent here has a playroom where we can sign in our daughter and leave her there for an hour while we shop, and they also have those nifty carts shaped like trucks so the kid can "drive" through the store. Shopping with the kid isn't that much of a headache, really, for us!