There's a great video about backseat safety for babies and preschoolers at this link:
and it talks about how to remember when you've left your kid in the carseat, especially on a scorching hot summer day. They recommend you leave your cell next to the kid, so you remember to look behind you when you get out!
I've locked my kid in the car by mistake, and left him there for an hour while I looked for AAA to help me unlock the car, but luckily that was in winter.
Anyone else as forgetful as me?