A cumulative update is a Windows update that contains a bunch of previous updates bundled as one. Its useful when you havent updated in a while because it lets you jump to a certain Windows milestone (like the Anniversary Update) without installing those past updates one by one. Unfortunately, cumulative updates can be tricky as evidenced by Cumulative Update KB3189866, which, according to user reports, can get stuck at numerous points throughout the update process namely at 45%, 46%, 48%, 49%, and 95%.