Honestly, we had our first child 15 months ago. We are both in our 30s and consider ourselves fairly well-educated. We've NEVER forgotten the kid in the back, even when he manages to fall asleep. In fact, we've felt "weird" when he wasn't in the back seat because he was with Grandma or the babysitter.
I'm not saying accidents don't occur every now and then, but to forget the baby in the backseat? Seriously?