Ok, we all understand the idea of a tombstone (a "deleted" field in the record that is set on deletion). However, the article actually describes something much

Ok, we all understand the idea of a tombstone (a "deleted" field in the record that is set on deletion). However, the article actually describes something much worse -- user deletes their account and subsequently the account is still active. Not "marked deleted but some data is still on the disk" but rather "completely active, works, can still log in". This is surprising to me. I would not expect to see that really ever. But the article claims that this occurred in some significant number of their tests. Not sure how to interpret that.