OP, I think it's normal and developmental. My DD1 is 7 and still gets distracted and forgets things that she knows she's supposed to do from time to time. Drives me batty! Simple things like not putting hair brushes away where they go (sounds minor, but she's lost three...!), not remembering things that she knows are 'rules'.

It does pass, on the whole, DD1 is now very responsible and can be trusted to look after things pretty well for her age.

I think consequences really do have to fit the behaviour itself. I have always tried to apply this rule (although sometimes I get really stuck and it's a pretty imaginative link to the consequence!). Some things are just memory issues, like Heaven said. They get distracted or excited and just forget your previous instructions. I'd just reiterate why the rule is so important - but really make sure you are stopping, getting at her level and as much as possible showing her why it's dangerous/not a good idea. So in the car, show her how far down she would fall and what she could hit her head on, explain how sometimes a car might need to stop very suddenly and you really don't want her to get hurt, etc. It's hard to be patient when you're getting home/driving, but looking her in the eye and talking calmly might just reiterate it better.

DD2 could undo her seatbelt at 6 mths. I can see a lot of boundary pushing in her future...