Facebook has admitted a glitch in its network meant people who had been blocked were temporarily unblocked.
The bug, revealed by the company on Monday, affected at least 800,000 people.
It was active between between 29 May and 5 June and applied to both core Facebook and its Messenger app.
The company said it would be informing those affected with a pop-up message, starting on Monday.
“While someone who was unblocked could not see content shared with friends, they could have seen things posted to a wider audience, for example pictures shared with friends of friends,” said Facebook’s chief privacy officer, Erin Egan, in a blog post.
“We know that the ability to block someone is important - and we’d like to apologise.”
The blog post details how no friendships that had been completely severed were reinstated. Of those affected by the glitch, 83% had “only one person they had blocked temporarily unblocked”, Ms Egan wrote.
While only affecting a small percentage of Facebook’s 2.4 billion users around the world, the glitch makes troubling reading for those who use the blocking feature as a method of escaping harassment on the platform.
The disclosure comes against a backdrop of a continuing privacy crisis at the company. Late Friday, the firm disclosed 110 third parties that had been given greater access to personal data - exposing a wider network of access than previously realised.