What is the most heartbreaking thing you ever saw?

I was once in a grocery store and there was a family with two rambunctious children. One was maybe six, a girl, walking beside the grocery cart and the other was a boy, maybe two, inside the grocery cart and playing with a toy. For some reason, the boy thought it was a worthwhile endeavor to throw the toy out of the cart and onto the ground, and so he did. Approximately half a second later he realized that he no longer had the toy, and the look of heartbreak on his face was traumatic even for me.

When his sister began to laugh at him, however, I did in fact experience a holy combination of amusement and despair.