Myles, I know a grandpa who left the house forgetting he had the baby sleeping in the crib and he watched the child often. I know another mom who went into the store forgetting the had a baby in the car...she almost always had a child or two with her...thankfully it wasn't too hot and wasn't a long shopping trip. I know another mom who forgot she had a baby a few weeks earlier and left the house with her husband and took a long walk.
I think forgetting a child, generally a sleeping child is much more common than we think. It's just thankfully it doesn't usually end tragedy.