Although most people realize that overstaying their non-immigrant visa status is a problem, they often misunderstand how big an issue it can really be down the road. Once you overstay the amount of time allowed on your I-94, you are considered to be unlawfully present in the U.S. The general rule on visa overstays is that if you are unlawfully present in the U.S. for more than 180 days but less than 1 year consecutively, and you then voluntarily decide to leave the U.S. and attempt to re-enter on a new visa, you will be barred from admission for a period of 3 years. Further, if you are unlawfully present in the U.S. for 1 year or more consecutively, and you then voluntarily decide to leave the U.S. and attempt to re-enter on a new visa, you will be barred from admission for a period of 10 years. These are obviously very serious issues and can have a major effect on a person's future ability to work in the U.S. or even to enter merely to visit.
Additionally, once you have overstayed your visa you will need to leave the U.S. and apply for a new visa abroad if you wish to enter the U.S. on a valid visa again. This often creates problems because a person who has overstayed for over a year for example, who then wants to leave the U.S. to enter on a new valid visa is exposing themselves to the ten year bar upon attempting to re-enter.
The United States is even taking measures to automatically detect when people overstay their visa timeframes.