Off your download for now and re-click on the torrent. Then let your BT check for the existing data, then when it resumes, it should finish instantly (since your at 99.9%). If you do that but the your error occurs again, leave the download on for awhile. Then if it doesn't finish, you can restart the entire download if you want. The thing is that even though it stops at 99.9%, your download is "technically" done, it just doesnt say File Complete!. So its up to you, you can restart the entire thing or consider your download done.