Your probably going to get a lot of anecdotal replies.
My Mom accedently left me at the grocery store when I was two. I think she expected me to get in the car on my own, and I didn't. She didn't realize I wasn't there until a few miles away, my brother asked, "Where's 'The_Victor?'" Even the best parents can have a moment of absentmindedness. We are human.
Well I have one too about my mom. When she was a baby she walked away from her parents at a beach (this was in the late 1920's) and she was found walking along a highway (of course there was hardly any traffic in those days) having fun happily playing. Needless to say my grandparents were frantic by that time and had the whole police dept out looking for her.