I'm having a serious issue, same problem that this person has from another forum found on Google.
It seems that if I disable "Add .bc! file extension to unfinished file" option from Bitcomet while I'm downloading an entire Season of a random TV show, the program will make previews of thumbs from other TV shows, documentaries or even movies that were deleted months ago and to be more precise 4-6 months. The thing is that it can play those videos even 2h length movies Bluray (6-10GB) on a 700MB file video with the same quality. Sometimes the previews that Bitcomet makes takes from other videos that I still inside my PC and can be also played. I tried to fix this by resetting Bitcomet settings, but it does it again by choosing other videos that they no longer exist in my PC.
However, once the video is complete from download it will play the episode from the actual TV-show I've downloaded a minute ago. Is like Bitcomet makes previews from old stuff until the torrent is complete. I don't want to use .bc option to "mask" this issue because I want sometimes to see the video while downloading or open .rar / .zip files directly and not waiting until is finished for the .bc extension to vanish.