As a result, over 800,000 users were subjected to at least one of the person on their blocked list being temporarily unblocked. With this bug, some purportedly blocked users could have seen posts that were shared with a wide audience and been able to contact the person who blocked them via Messenger.
Rather than wait for some enterprising third-party security researcher to unearth the bug, Facebook's chief privacy officer, Erin Egan, took a swig from the mea culpa chalice. Also, the bug would have let these individuals re-add friends who had blocked them from their profile.
According to the company, the bug was caused by a "technical issue".
When a user blocks someone on Facebook they can not see things posted by the user, start conversations with the user on Messenger or add as a friend.
Mind you, they couldn't see private posts, or posts you'd only shared with your friends.
That ex you blocked on Facebook may have had access to your posts after all. "We know that the ability to block someone is important and we'd like to apologise and explain what happened", Egan added.
Facebook apologized for the error and is encouraging users to check their blocked lists.
Now Playing: Watch this: Facebook bug changes 14 million users' settings, Amazon. The bug deleted these "associations", which hit the block lists of Facebook and Messenger for some users.
In its defense, Facebook says that the vast majority of people affected by the bug - some 83 percent - "had only one person they had blocked temporarily unblocked".