Facebook has just disclosed that around 800,000 users were impacted by a bug that silently unblocked “some” people they had blocked.

The bug was live from May 29 until June 5, the company says.

Worth noting: The bug didn’t go so far as to make the would-be blocked individual your friend (even if they were your friend prior to the block), so anything an affected individual might’ve posted to a friends-only audience should have remained private. It would, however, allow a blocked user to do things like contact you on Messenger, or try to re-add you as a friend.

While Facebook isn’t saying much about what caused the bug, they’re sending out a notification (pictured above) to anyone they believe was affected.

Update — Facebook has shared some more details about the cause of the bug with TechCrunch’s Josh Constine via Twitter:

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