Its harmless since it doesn't get you stuck or anything (you can still get out of it and drive the cart normally) but if you jump over the shopping carts at the Costco skate park and press y while you are above them you can hop in while doing the aerial animation and slowly sink below the level.
Haven't tested it on the shopping carts at the other levels, but tried it with the ATV there and it doesn't cause the same bug.