Facebook said it is notifying more than 800,000 users that a software bug temporarily unblocked people at the social network and its Messenger service.
"We know that the ability to block someone is important," Facebook chief privacy officer Erin Egan said in a blog post on Monday.
"We'd like to apologize and explain what happened." Blocking someone on Facebook prevents them from seeing posts in a blocker's profile; connecting as a friend, or starting Messenger conversations.
Blocking someone also automatically "unfriends" the person.
"Their relationship may have changed or they may want to take a break from someone posting content they find annoying." People are blocked for harsher reasons, such as harassment or bullying, Egan added.
The software bug did not restore any severed friend connections at the social network, but someone who was blocked could have been able to reach out to a blocker on Messenger, according to Facebook.
The vast majority of the more than 800,000 people affected by the bug had only one person they had blocked become temporarily unblocked, according to Facebook.
Facebook chief Mark Zuckerberg earlier this year was grilled by the European Parliament and the US Congress about a massive breach of users' personal data in the Cambridge Analytica scandal.
Facebook admitted that up to 87 million users may have had their data hijacked by British consultancy Cambridge Analytica, which worked for US President Donald Trump during his 2016 campaign.
(At The Quint, we are answerable only to our audience. Play an active role in shaping our journalism by becoming a member. Because the truth is worth it.)