That's the painful part of it, Katydid. That's the haunting part. I don't know how - it certainly never happened to me.
But when I see something that I've seen before - this out of sight out of mind thing - I believe it can happen. I believe it.
I've mentioned this numerous times, but I had a boss who this nearly happened to. On her way to work, she arrived at the babysitter's house and the babysitter had something going on (a child was sick, she was sick, something) so my boss decided to drive to work, park, grab a few folders and items and go home for the day to work from home.
She parked her car in the parking lot, locked it, and sprinted in to the office to grab things. We were having a crisis that day. At that point, my boss swung into crisis mode and FORGOT her baby in the car in the parking lot.
She had been at the babysitter's office five minutes prior. She had walked into the office with full intent to get items that she had listed in her head for a day at home, and within one minute that was forgotten and she was in crisis mode. This was August. After at least an hour, she suddenly screamed "MY BABY!!" from in her office and ran out to the parking lot, where by the gift of God, he was fine. A sweaty mess, but alive and well. By the grace of God she had the one parking space in the lot that was shielded by this enormous dense shade tree.
I can't forget that Katydid although it's been 30 years. I saw it happen, and for those minutes believed the child would likely be found dead in the car.
Because inexplicably, this woman I really love and admire completely forgot in a matter of a few minutes her plan to take her baby back home and work from home that day, and forgot he was in the car.
It wouldn't happen to me, but I've seen it in someone I care about.