Source: Facebook

Facebook disclosed a new “bug” on Monday that temporarily let some users who’d been blocked on the service send messages to the people who had blocked them. The bug also let some previously-blocked users view posts that were shared “to a wider audience,” such as publicly or with friends of friends, Facebook said.

Facebook’s privacy boss Erin Egan apologized for the error, writing in a blog that the company is reaching out to “over 800,000" users about the screw-up. The “blocking bug” was active between May 29 and June 5, for eight days, though the company now says Messenger should be acting normally.

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Egan’s post details the features of this newly disclosed bug.

  • It did not reinstate any friend connections that had been severed;
  • 83% of people affected by the bug had only one person they had blocked temporarily unblocked; and
  • Someone who was unblocked might have been able to contact people on Messenger who had blocked them.

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It isn’t clear when Facebook discovered the bug or how many people were actually contacted by the people they’d blocked. It’d be interesting to know if Facebook discovered the issue itself, or after users complained about unblocking. We’ve reached out to Facebook for more information.

Facebook’s announcement post closes by mentioning the serious consequences of a faulty blocking system, chiefly harassment or bullying, both significant issues that are exacerbated when platforms make mistakes like this.