I'm not breaking any laws. Then, I keep doing what I've been doing. Suddenly, I am breaking the law. A short while later, still not alteration to my behavior, I'm not breaking any laws again.
Now, to make it harder - it's crucial and important that the action is something that most of us do, have done and will keep doing. There might be a few exceptions but I'd dare to say that (almost) everybody knows about it and is friends with someone who does it. It's a very common thing and one can see it pretty much everywhere and all the time.
How is it possible? What's the action?