The development workflow of Drupal core is patch-based. This means that sometimes, while the patch is evolving on the issue, core changes and the patch does not apply anymore. When someone detects that this is happening on a given issue, she must tag it with the "Needs reroll" tag.

You can find the list of bugs tagged at https://www.drupal.org/project/issues/search/drupal?issue_tags=Needs%20reroll&categories%5B%5D=1