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Dundas: It seems like you were refused entry ONLY because the processing of TNs was done during normal business hours. FYI, the company I work for had a few TN candidates denied the visa due to insufficient proof of qualifications, but at least one candidate reapplied the next day with necessary documents and there was no issue. In that regard, having to reapply for a TN (come back next day or bring additional documentation) didnt harm the applicant\'s chances.
When you are asked about any "refusal of entry" by the Consular officer, you can mention the context, stating they asked you to come back the next day during normal business hours, you did, and the TN was issued without a hitch. The officer probably will be satisfied with this explanation. You werent refused for legal reasons, just a practical one (office closed).
What the INS/State Dept are looking for with this question are any instances you were denied entry due to suspicious behaviour, previous criminal records, intent to work without a valid visa, fraudulent intent of entry etc. If you have not had any issues in this regard, you should be fine. Of course, you should get an opinion from your lawyer on this, but I wouldnt worry if I were in your situation.