According to Erin Egan, Chief Privacy Officer at Facebook, 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.
Normally, 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. Facebook said that 83% of people affected by the bug had only one person they had blocked temporarily unblocked.
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.