Edit: I wasn't expecting this to get so much attention, so I thought now might be a good time to add: don't put too much stuff in their cart or things that are too expensive... Be reasonably insane.
Did it, we put some...very personal items in someones shopping cart and proceeded to see if he noticed when checking out, he didnt, so he went home with something hilariously nasty
Ahhh, those people who leave their carts in the way for like 20 minutes? Hm, maybe if there's a baby in a cart, and the parent isn't looking at the moment, I can place it into the abandoned cart