The mistake of allowing the I-94 card to expire can have catastrophic results. In the United States, there are harsh penalties for failure to maintain status, as well as bars on reentry for those who have extended periods of unlawful presence.
Generally, once a person has fallen out of lawful nonimmigrant status, s/he is no longer allowed to extend or change status without leaving the United States. This procedure is expensive, inconvenient, and can be risky. More importantly, if one has overstayed the time allowed to remain in the U.S., her/his visa will be canceled. Renewal of the visa in that instance will require an application in the home country. One who has been unlawfully present for 180 days or more, will be barred from reentering the U.S. for three years from the date of departure. One year or more of unlawful presence triggers a ten-year bar on reentry to the United States. Counting the time one is unlawfully present is not always straightforward. Thus, if this problem is discovered, it is vital to get qualified immigration legal advice before making any decisions.