An interesting situation has arisen in England in the past few days regarding the Video Classification Act of 1984. This is the act that requires videos to be classified before they can be sold and that you can't sell videos to people who are too young to watch them.
It seems that because the government at the time didn't follow all the procedures required for it to become a law, the law never actually came into effect. This in practice didn't cause many problems because everyone was acting as though the law was real, and movies got classified and most people didn't try to sell porn to children, so all went well.
Even now don't expect an apocalypse of unclassified movies being sold to anyone. Businesses have agreed to follow the laws requirements on a voluntary basis until new legislation is passed later this year.
The tricky part though is that a number of people have been prosecuted under this law and found guilty and punished. The government has dropped all current prosecutions, but is claiming that existing convictions under the act can not be challenged. This is where I strongly disagree with them. If there was no law, there can not have been a crime. While it is regrettable, if we are to have the rule of law, then the convictions should be overturned.
This is also a reminder that there should be more scrutiny of the process of lawmaking. This law is not exactly the most important of laws, and fixing the problems that have occurred won't be too difficult. But imagine if something big were to be found not to be a law. Like murder, or even worse, taxes. Imagine what it would do to the government if it were found out that there was no tax law for 25 years. Everyone would jump on that demanding their money back. The government would be so screwed.
There should also be more active review of laws. This mistake was picked up by the Digital Britain scheme, which is something to do with improving internet access in the UK. If there had of been someone regularly reviewing the existing laws it may have been picked up earlier.