In yet another privacy goof-up, Facebook has admitted that over 800,000 users were affected by a bug on its platform and Messenger that unblocked some people these users had blocked.

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, said Facebook.

"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," Erin Egan, Chief Privacy Officer at Facebook, wrote in a blog post on Tuesday.

When you block someone on Facebook, they cannot see things you post on your profile, start conversations with you on Messenger or add you as a friend.

Blocking also automatically unfriends them if you were previously friends.

"In the case of this bug, it did not reinstate any friend connections that had been severed," Egan said.

Nearly 83 per cent of people affected by the bug had only one person they had blocked temporarily unblocked.

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Someone who was unblocked might have been able to contact people on Messenger who had blocked them, Facebook noted.

The company said the issue has now been fixed and everyone has been blocked again.

"People who were affected will get a notification on Facebook encouraging them to check their blocked list," the company said.

Facebook has already been grappling with privacy issues like the Cambridge Analytica data scandal involving 87 million users and another bug that changed 14 million users' privacy setting defaults to public.

"While 800,000 people is just a tiny fraction of the 2.2 billion Facebook user base, that is still a sizable number of affected users who may have been subject to concerning episodes," The Verge reported. (IANS)