"We know that the ability to block someone is important", Egan wrote, "and we'd like to apologize and explain what happened".
Facebook's blocking feature enables users to restrict certain individuals from viewing anything on their timeline, tagging them in any post, or starting conversations with them on the Messenger app.
The bug was live from May 29th until June 5, the company says.
Responding to Facebook's blog, TechCrunch's Josh Constine said in a tweet that the block bug would further shake the confidence of stalking and abuse victims who trust the platform. But if you sent a post to "friends of friends" or a wider audience, blocked people could see them.
However, the previously blocked person could add as a friend or even contact on Messenger.
Facebook has revealed via a blog post that a bug, active between May 29 and June 5, had messed with over 800,000 users' block list over that one-week period, accidentally unblocking people the affected users had previously blocked. This will come as little comfort those anyone who found (or, indeed, is just now learning) that people they purposely blocked was able to see their posts.
People are blocked for harsher reasons, such as harassment or bullying, Egan added.
"Their relationship may have changed or they may want to take a break from someone posting content they find annoying". Affected users, nevertheless, are going to get a notification encouraging them to review their blocked list.
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. Personal and sensitive data of users was scraped by the analytics company which was allegedly used for Trump campaign for the United States presidential polls 2016, among others.