Yesterday (November 19), a "technical fault" meant that a voucher code worth £15 could be used by every customer.
However, any orders that were placed with the discount active will not be honoured by the online retailer.
Play.com emailed affected customers earlier this afternoon: "On the 19th of November, you were one of a number of customers to use a £15 voucher code at check-out on our site.
"Our sincere apologies, but there was a technical fault with this offer, which meant that a voucher intended for a single customer could be used multiple times in error.
"As a result, all subsequent orders could not be processed.
"As a valued customer, we would like to apologise for any inconvenience caused."
> Play.com bought by Japanese firm Rakuten for £25m