Gnutella Forums - Overlaping video files

I have had several files that I have either already downloaded (in my shared folder) or files that I am in the process of downloading overlap each other and become corrupted. How does this happen?? Now the files are useless and I have to re-download. I know that the files are not corrupt because I have watched the completed ones. One complete video actually combined with another and now that file is corrupt. Also, the file size of the incomplete videos are now double what they were originally listed as. One of the files is now stuck at 99% and wont stop downloading! Please help!