I was in a grocery store a couple of weeks ago, and this woman's dog crapped through the grate of the cart onto the floor right in front of the deli. She was just walking away until I called her on it. She pretended not to have seen it and asked how I knew it was her dog.

I flagged down an employee, but didn't say anything, I just stared at her until she did.
Kind of an asshole move, but I wasn't trying to make her feel bad, just do what any reasonable person would do. Also, don't bring your freakin' dog into a grocery store unless you're blind. Also, if you're going to be an asshole and do it anyway, at least make sure the thing craps BEFORE going in!
They cleaned the floor, but what about the grates of the cart? I hope the remains of dog shit got all on her food, and she didn't notice it in a small amounts on her hands before she absent mindedly licked off her fingers.

No, I don't hope that. I hope she learned a lesson and doesn't do it anymore. But I kinda wish I hoped something harsher!