Facebook issued an apology on Monday to more than 800,000 users it says may have been affected by a bug that unblocked people the users had previously blocked.
The company said in a post that the bug was active between May 29 and June 5.
Users on Facebook can "block" others as a way to prevent them from seeing posts or contacting the user on Messenger. With this bug, some purportedly blocked users could have seen posts that were shared with a wide audience and been able to contact the person who blocked them via Messenger. (They could not see content that users who blocked them shared with just their friends.)
Facebook said the problem has been fixed. Users will be notified if the bug was linked to their accounts and will receive a Facebook notification "encouraging them to check their blocked list," the company said.
It's the second software bug the company has acknowledged in less than a month. In June, Facebook disclosed that a software bug set some posts to "public" by default regardless of their previous settings. That bug affected as many as 14 million users over several days in May.
The disclosure is the latest recent in a long-running bout of privacy blunders by Facebook, which is still reeling from March revelations that political consultancy Cambridge Analytica had accessed information on some 87 million users without their knowledge or consent.
The Associated Press contributed to this report.