I admit with regret that I jumped in with bashing her after the photo of her stumbling with him. I agree with those of you who have said that the only real, absolute no-no was her driving with him in her lap.
The nanny was the one who dropped him; the rear-facing vs. forward facing car-seat doesn't seem to be a concrete law; the convertible probably wasn't the safest or smartest thing but she wasn't on a busy freeway either, didn't it say it was her neighborhood so she probably wasn't going very fast; as far as the possibility of him getting sunburned, yeah it happens quicker for babies than adults but she could've been in the car 10 minutes, maybe just trying to get him to fall asleep. I can't count the number of people that have used that guaranteed nap inducer.
Anyone can stumble and she doesn't have to wear sneakers just because she has a baby. Parenthood can be hard for anyone but knowing the world is watching and judging your every move,for anything to blow out of proportion, I can't imagine such stress.
So on the slim chance that you're reading this Britney, I apologize for being so overly critical. Dump KFed and get rid of his negative energy and focus on making your kids and your life better.
ETA: IMO, she still says/does other incredibly immature stuff, my opinion of that stuff hasn't changed. (Like that picture on her website, giving the finger.)