Probably, with a 5 month old baby at home, you don't sleep a lot. It has happened to me once taking the way to the office and then having to come back to the baby sitter's house to drop off the kid. It happens. I think a good idea is to put the kid's seat in the middle seat. That way you see him every time you check the rear mirror. That saved me that time.
Or have the briefcase/purse/bookbag always next to the car seat.