Starting today we are notifying over 800,000 users about a bug in Facebook and Messenger that unblocked some people they had blocked.
The bug was active from May 29 to June 5 in both Facebook and Messenger, company chief privacy officer Erin Egan informed. About 83% of those Facebook users had one person they blocked essentially unblocked by the bug, while 17% had more than one blocked person unblocked during that time period, the social network said.
"We know that the ability to block someone is important - and we'd like to apologize and explain what happened", Egan wrote.
The fact that this was possible is a concerning breach of Facebook users' online safety.
Facebook confirmed that a bug has temporarily unblocked some users from the block lists of around 800,000 users.
The bug would automatically unblock users that you may have blocked on Facebook, meaning they would be able to view some of your posts.
This comes a month after Facebook disclosed a software bug, which, for several days in May, changed the suggested audience of all new posts to "public".
The bug did not reinstate any Facebook friendships, and the majority of those affected only had one person unblocked.
In its original statement, Facebook pointed out why someone might want to block another user.
People are blocked for harsher reasons, such as harassment or bullying, Egan added. But between May 18 and May 27, some 14 million people made public posts on the social network, many of them likely believing they were sharing with just friends.
It's not on the scale of the Cambridge Analytica scandal, but Facebook once again has egg on its face thanks to a privacy problem.