Every year, it happens: someone forgets that their kid is in the car on a hot day while they go inside and do something, and the kid dies of hyperthermia. And every time it happens, the chorus comes: "How can anyone do this? What kind of awful person does this? How is this even possible?"
Here's some insight into this phenomenon.
This is, as one might expect, a gut-wrenching story to read. Beware.