An I-94 is the Customs and Border Protection (“CBP”) form used to track a foreign individual’s U.S. arrivals and departures.  U.S. Citizenship and Immigration Services (“USCIS”) uses Form I-94 also. In most instances, Form I-94 must be completed at the time of entry to the United States by foreign citizens who are being admitted into the United States in a nonimmigrant visa status.   It is the individual’s responsibility to always review the I-94 card that is issued to him/her each time they enter the U.S.  Incorrect information on an I-94 card, even incorrect information due to an error by the Immigration Inspector, can have very serious immigration consequences.  
Case Study:
Scenario: Mr. Dornan petitions USCIS for O-1 nonimmigrant status and USCIS approves the petition with validity dates of October 17, 2012 through October 16, 2015.  Upon entry, an Immigration Inspector erroneously issues an I-94 card with an expiration date of October 16, 2014.  Mr. Dornan remains in the U.S. and never looks at his I-94 card again until July 27, 2015 when he is ready to extend his O visa.  Does Mr. Dornan have an immigration problem.
Response:  Yes.  Overstaying the I-94 expiration date, even if the overstay was inadvertent, by 180 days or more bars readmission to the U.S. for three years.  Mr. Dornan’s I-94 card expired on October 16, 2014 regardless of the validity date of his USCIS Approval Notice.  This means that as of July 27, 2015,  Mr. Dornan has already overstayed approximately 284 days.  Should he continue in a period of unauthorized stay for one year or more, he will accrue a bar to readmission for ten years.  Overstaying the I-94 date by even one day can result in the individual being subject to a requirement of having to return to their home country to obtain all future nonimmigrant visas.
Solution: The U.S. Immigration law is complex & confusing.  It is critical to have your case and related documents reviewed by a skilled immigration team.
Page Summary:  Even inadvertent errors can lead to significant immigration consequences.