[Modernman]

There are SO MANY millions of laws nowadays that no one can possibly be aware of them all, let alone obey them.

Ignorance of the law is no excuse.

[exmarine]

Ignorance of the law is no excuse.

That's a good justification for persecution, as if tyrants need one.

[zeugma]

Ignorance of the law is no excuse.

That will be a great comfort to you when you are driving through Texas and you get a ticket for having a license plate holder on your car that obscures 1/3 of the top of the letters of the name of whatever state it's registered in.

Note that it doesn't matter if you can clearly distinguish the state of origin from the plate itself. If just a tiny bit of the lettering is obscured in any way, you can be ticketed.

[bvw]

While that maxim "Ignorance of the law is no excuse." is still the rule, it is less and less of a good rule for the reason given you. It is being questioned in legal circles, in legislation discussions, in discussions of regulation law.

Here is one example from CATO:

The sheer volume of modern law makes it impossible for an ordinary American household to stay informed--and yet the U.S. Department of Justice vigorously defends the old legal maxim that "ignorance of the law is no excuse.'' That maxim may have been appropriate for a society that simply criminalized inherently evil conduct, such as murder, rape, and theft, but it is wholly inappropriate in a labyrinthine regulatory regime that criminalizes activities that are morally neutral. It has been estimated that the number of new enactments by legislative bodies ranging from city councils to Congress is 150,000 per year. At that rate, a conscientious citizen would have to study 410 laws each and every day all year long--a full-time task, to say the least.

Source: http://www.cato.org/pubs/handbook/hb105-22.html