Did you ever watch an incomplete video download? Under Linux there’s no problem with that. I’ve heard that Windows doesn’t allow two programs to open the same file, so you can’t download and preview in the same time, unless the downloading program saves a copy of the file somewhere.
For example, an incomplete Bit Torrent file has a full size, but a lot of small and big gaps in the data. The video codec tries to play the file and when it encounters a gap, it skips to next readable part of the file. What happens on the screen is that there is the before-the-gap frame to which the after-the-gap movements are applied. So you see a forest which suddenly becomes a blanket on someone’s head… and then the person blinks and the eyes pop out. Now it’s just garbled, unfinished file, but I’m sure that someone, someday, will get inspired and use this kind of distortion as a legitimate special effect.
UPDATE: Since there are people who come across this post looking for a way to watch incomplete files, here’s a little information for them.
There are many p2p programs, and this is your program-specific. The most important question is: Where does your program store temporary files? I use MLDonkey, which supports many p2p networks, including EDonkey and BitTorrent. In my installation, MLDonkey keeps the incomplete files in /home/p2p/mldonkey/temp directory. The files, while being downloaded, have temporary filenames, full of codes and digits. When there are many of them, it’s hard to tell, which is which. So I just try to open each one with MPlayer.
If you use another p2p software, I can’t really help you, beside telling that you should look for a “temp” or “tmp” directory somewhere near the installation of your program. It probably won’t be /tmp (Linux), C:\tmp\ nor C:\windows\temp (Windows). And besides, Windows will probably keep the files locked.