However, Xiaomi denied accusations this indicated no phones had been sold at the promised price.
Instead, its UK sales manager Wilkin Lee said, a lottery had taken place made up of those who had pressed the button closest to the set time.
"Of the thousands who clicked 'buy' simultaneously, the tie-breaker is done by selecting the winners randomly," he tweeted, external.
But users have complained the terms and conditions made no mention of a lottery, stating only that the phones would be "given away on [a] first-come, first-serve basis".
Moreover, one user pointed out at one point the text had not stated how many phones would be offered, external.
A spokeswoman for Xiaomi has confirmed that this was the case but added that this "mistake" had been fixed the day before the first sale.
This could still have consequences as the Advertising Standards Authority's rules state that consumers must be informed, external "clearly and in a timely fashion" about the limited availability of a product.
"If the ad didn't include significant conditions and the terms and conditions were changed part way through the promotion, then that could potentially be a problem," said a spokesman for the ASA.