Facebook sent out a press release that there was a bug in the system on Monday that caused blocked users to become unblocked in Facebook and Messenger.

According to the release, this bug was active for 800,000 users between the dates of May 29 and June 5.  The Chief Privacy Officer, Erin Egan, explained, "... while someone who was unblocked could not see content shared with friends, they could have seen things posted to a wider audience."

Ordinarily, when you block someone on Facebook, you block that user from seeing your posts on your profile, they can't start conversations with you through the Messenger App and they cannot add you as a friend.  It also causes you to unfriend them if you were previously friends with them on the platform.

The release also mentioned that everyone affected have already been contacted by Facebook, the bug has been fixed and those previously blocked have become blocked again.

Check out the release here for more information.