The retailer's stock-management system failed to provide adequate information on stock levels over the weekend of 16 and 17 July, when B&Q offered 75% discounts on some product lines.
The large discounts, aimed at freeing space in B&Q's central warehouse, resulted in stock running out rapidly, with the company taking about 1,000 orders for stock it did not have.
B&Q's director of business systems, Mike Sackman, said, "The system that checks on how much stock we get is not real-time enough in this area. There's obviously a review we need to do."
B&Q's director of supply chain development, Richard Houlton, said problems with the stock checking systems were "compounded by a systems problem with our credit card authorisation system." The B&Q card payment system failed after the installation of a new piece of software, said Sackman.
All those customers who paid for goods that B&Q no longer held in stock have been refunded and have received a gift voucher.
Sackman would not disclose precise details of the systems that had caused problems. Its IBM Consumer Demand Management Service with DemandTec had not been set up to check stock levels of heavily discounted products sufficiently frequently.