I've recently been receiving a huge amount of email asking about AVIs that people have downloaded which seem to have the video encoded using FOURCC 0 (0x00000000). Needless to say, these files will not play and, since most seem to be ripped off movies, the folks who spent hours downloading them are less than happy.
FOURCC 0 is actually defined. If you look on the RGB page, you will see that it represents standard Windows bitmaps. This format is completely useless for video since, although you can guarantee that there is no image degradation when you save this way, you also guarantee no image compression at all and, hence, an enormous file.
If I was a conspiracy theorist, I would suggest that these corrupt files may have been deliberately planted by copyright owners to waste a lot of people's time (to be perfectly honest, I rather like this idea - if you want to steal movies, you deserve some of this kind of hassle, IMHO) but I really don't know where they come from and don't know whether they are playable if you use some tool to change the FOURCC to something else.
I am told that many of these files are merely MPEG movies which have been mislabelled as AVI. If you change the file extension to .MPG, apparently about 50% of them will play correctly.