We've all done it...
My Hubby and I have lost out son, twice, because we thought he was with the other, and more recently, lost our little girl once, because we thought she was with the other (but she was really with my son, she's 8 and he's 14, so no real danger there, just scared to death parents!).
My Mom had me, my sister, and my first brother all within a five year time period, when my brother was two weeks old, my Mother was exhausted, had "Mommy Brain", had to bring the three of us to an appointment, and then left... forgetting that she'd already had the baby and that she'd just left him in his carrier on the waitingroom floor! Apparently, we caught the bus, she realized what she'd done, we got off at the next stop, and she did the marathon run with her then five year old and 2 1/2 year old daughters, all the way back to the office, to find about 30 people oohing and ahhing over this newborn baby boy, everybody passing him around, trying to figure out who's baby he was. Can you imagine how embarrassed she must've been?!
One of my co-workers, an older woman, likes to tell the story of, when her sons were four and two years old, they were traveling by commuter train and she was carrying her two year old son and holding her four year old's hand, and she let go for half a second, but told her four year old to stay close. They got on the train, the train started to pull away, when she hears the little one, waving out the window and saying, "Bye, bye Omar.". Omar (the four year old) was on the train platform, smiling and waving at the leaving train!
Can you imagine? It happens to all of us.