By Erin Egan, Chief Privacy Officer

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 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. We know that the ability to block someone is important — and we’d like to apologize and explain what happened.

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; 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.

This issue has now been fixed and everyone has been blocked again. People who were affected will get a notification on Facebook encouraging them to check their blocked list.

There are many reasons why people block another person on Facebook. For example, their relationship may have changed or they may want to take a break from someone posting content they find annoying. Other reasons are more serious like harassment or bullying.

For further information about how to block someone on Facebook, or for more information about how to deal with bullying or harassment please visit facebook.com/safety.