If a stop time pair for a cell becomes globally visible, we destroy the key in reconciliation. When we do this, we need to begin clearing out history store content for that key.

Imagine a case where we have timestamps 1, 2, 3 in the history store and 4 on the page itself. If you apply a non-timestamped tombstone, you will destroy the key next time you reconcile the page. However, until the next checkpoint that history content will remain there. Now let's follow up with an update at timestamp 5 to recreate the key. Now if we do a read at timestamp 4, the onpage value (5) won't be visible to us so we'll go into history store and read timestamp 3.