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I've had this happen to me a couple of times. For example, the power went out and my computer crashed catastrophically. As per Duki, the torrent client didn't get a chance to send out it's terminate signal. Most of my torrents were still "Active" four days later when I got back online. No damage, just annoying.
Normal Windows shutdown can do the same if Windows terminates an active client session because you forgeot to turn the client off gracefully. I've fixed this in the past by activating the torrent client & the torrents in question, then Pausing &/or Stopping them. Terminate signal gets sent, tracker catches the update within ~10-30 minutes.