McKinnon v. The Queen, 2004 DTC 2049, 2003 TCC 884 -- summary under Subsection 227.1(3)

The failure of the company to remit source deduction was due to an unforeseen supervening event (the arbitrary improper refusal of a major customer to pay amounts due by it to the company), and it was unreasonable of CCRA to take the position that the taxpayer, as sole director of the company, had showed a lack of due diligence when he failed to terminate the business upon the commencement of cash flow problem. Accordingly, the due diligence defence was made out.