I'd just arrived at the park with my dog. Two people ahead of me had a toddler and a Westie on one of those extending leads. The dog had lagged behind them and crouched to go to the toilet. They weren't aware. I wasn't sure whether it had done a poo or a wee but when I drew level with where it had crouched, there was a fresh pile there.
I called over to the people "excuse me, your dog has just been to the toilet".
The woman called back "not our dog, he went in our garden before we left".
I said, "sorry he's been again, it's just here".
The man came over and repeated that it wasn't their dog. I said "it was, I saw him go". At which point, I said "don't worry, I'll pick it up".
I cleared up the dog poo and it was still warm. I offered the man a feel but he wasn't impressed. 
A woman with a labrador had heard the exchange and she said to me it would now be karma if their toddler trod in some poo.
I was really annoyed, quite honestly. I would be delighted if someone told me that I'd missed my dog going but this couple couldn't have cared less.
My DH said I should have given him the poo.