What are examples of government not enforcing laws?

I need examples of when the government did not enforce laws and were incapable of providing safety for its citizens that vigilantes had to come into play? For instance, the September 11th example where the "vigilantes" on the plane tried to take control back from the terrorists on Flight 93 so it couldn't crash into an important building?

There are plenty of cases of good old boys at the local government level not enforcing the laws even when they know that someone's health and safely are at risk , or of their selectively enforcing the laws in or der to provide the greatest financial advantage to themselves and their friends. Ask around in your city or town.