Failure to turn in your I-94 when you leave the U.S. after the expiration of your TN status can create a serious problem. Without this record of your departure, you may be identified in U.S. immigration records as an overstay. Being identified as an overstay means that you may be denied any future re-entry into the U.S.
If you have failed to turn in your I-94, U.S. Customs and Borders recommends sending the I-94 along with any documentation that proves you have left the U.S. to:
1084 South Laurel Rd.
London, Kentucky 40744
Documentation that may be submitted to prove your departure from the U.S. can include the boarding pass from your flight, copies of your flight itinerary, credit card payments made outside the U.S., etc. If you exited the country by a land border it is more difficult to verify that you did in fact leave the country on the date you claim. If you have any documentation of your arrival in your home country (e.g., a passport stamp), then you should send a copy of that.
If ACS Inc. does not receive supporting documents that substantiate your claim to have left the U.S. on a certain date, there is no guarantee that you will be entered into the record as having done so. CBP strongly urges individuals to keep a copy of all materials sent to ACS Inc. and to carry it with them the next time they come to the U.S. in case the CBP officer has any questions about eligibility to enter. CBP processes this paperwork in approximately four months. After this time, you may request confirmation that your departure was recorded by writing to ACS, Inc. at the address provided above.
For more information, visit the U.S. Customs and Border Protection site.