1. You download a file through some Torrent Client like BitComet, it shows 100% but you can't open the file.
2. There is .BC! file in your system after Torrent download, which you can't open simply by renaming.
Reason : Because of missing/corrupt file pieces ( technically IP packet corrupt/missing) you can't open the downloaded file even after it says 100%.
Solution : All the Torrent files are signed with a Hash key, which helps in identifying the integrity of the file. Also you can go to "piecegraph" the file is 100% finished but still green and red dots instead of all blue. This indicates the pieces are missing or corrupt.
Below steps apply to BitComet, but the same concept can be applied with other Torrrent Clients as well.
1. Once the Client shows 100% Right click on the Torrent Task
2. Select Manual Hash Check
3. Torrent Client checks the hash again with the original Hash key
4. If it finds Hash Key mismatch it will again Requests the missing pieces
5. After missing/corrupt file download the Client shows 100%
6. Now you can open it.
Hope this helps.