Over 800,000 accounts on the social media network were plagued by the hiccup that unblocked people users had blocked.
Facebook’s chief privacy officer, Erin Egan, disclosed the bug occurred between May 29 and June 5 in a blog post.
He said: “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 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. For example pictures shared with friends of friends.”
Although the bug did not add friend connections to any of the unblocked users, it could have allowed previously blocked users to have sent friend requests to the user that blocked them.
Mr Egan also stated the bug may have allowed previously blocked users to have contacted the person that blocked on then on Facebook’s Messenger service.
He went on: “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.
“Someone who was unblocked might have been able to contact people on Messenger who had blocked them.”
The privacy boss also disclosed 83 percent of people plagued by the hiccup only had one blocked user unblocked temporarily.
Although Facebook insists the bug has been fixed and all users that were previously blocked have been blocked again, it is still worth checking your account to make sure this is the case.
The tech giant has promised everyone hit by the bug will receive a notification on the social network telling them to check their block list.
When pressed on why the bug occurred by journalist Josh Constine, Facebook responded with a tweet attempting to clarify the situation.
The company tweeted: “It’s always hard to find the right level of technical detail to put in a blog post like this.
“More context on what caused the bug: most visible user data on FB is stored in pairs called ‘associations’ which control what posts people see and the actions they can take.
“A bug mistakenly deleted some of these associations across Facebook and Messenger, which caused blocks to be lost.”