As the recent earthquake and tsunami in Japan illustrate, there are times when natural events or other circumstances outside a person's control prevent that person from timely leaving the US. Flights can be cancelled or an accident can land the person in the hospital.
When this happens, it is important to document what happened, and to contact USCIS as soon as possible.
People affected by flight cancellations due to the crisis in Japan should keep a record of the flight cancellation. I would recommend also keeping at least some clippings of the news to show why the flight was cancelled. These people should also contact USCIS to obtain an extension of their stay, and if they are already at the airport when the flight is cancelled, should contact the Customs and Border Protection office at the airport.