Who May Not Apply for a Visa at an Consular Section in Canada or Mexico?
Individuals who have been out of status in the United States because they violated the terms of their visa or overstayed the validity indicated on their I-94 should not seek to apply in Canada or Mexico and should return to their home country to apply. In other words, if the traveler has remained in the United States longer than the period authorized by the immigration officer when the traveler entered the United States in any visa category, the traveler must apply in the country of the traveler's nationality or legal permanent residence.
http://usvisa-info.com/en-CA/selfservice/us_tcn_present_united_states
