A newsletter campaign can finish without sending a single email: the audience was empty, every contact was suppressed or unsubscribed, or every send failed. Until now it still got the same green completed badge as a campaign that went out normally, so nothing told you to look.
Those campaigns now show an orange Completed — 0 recipients or Completed — 0 sent badge on the Campaigns page and on a list’s Campaigns tab. The campaign’s own page explains what happened. When every send failed, the recipient list shows the error for each contact.