Facebook has apologised after a bug temporarily unblocked people that users had blocked on the social network.
The bug affected more than 800,000 people and was active between May 29 and June 5, the company said.
Facebook confirmed the issue had now been fixed and those affected were being notified.
Blocking can be used to hide from view posts by fellow users that a person no longer wishes to see, as well as to keep their own content hidden from specific accounts.
The social network said the bug did not reinstate any friendship connections which had been severed and 83% of people hit by the bug only had only one person unblocked.
"Starting today we are notifying over 800,000 users about a bug in Facebook and Messenger that unblocked some people they had blocked," Facebook's chief privacy officer, Erin Egan, said in a blog post.
"The bug was active between May 29 and June 5 - and 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.
"We know that the ability to block someone is important - and we'd like to apologise and explain what happened."
Blocking on the social network is a tool often used to help users avoid abuse or harassment on the platform, which is used by more 2.4 billion people globally.
Facebook's admission comes amid continued data privacy issues for the firm, which is still dealing with the Cambridge Analytica scandal, and also recently revealed a list of companies it gave special access to data - on a larger scale than previously disclosed.