OS: Windows XP SP2
Compiler: GCC under MinGW and MSYS
CPU: AMD Athlon 64 3000+ (Venice core)
Version: CVS of MPlayer and FFmpeg as of ~10:45am BST.

Source: Some 1080p 25fps clips resized to qcif for quick testing purposes.

Command line used:

mencoder.exe qcif.avi -ovc lavc -lavcopts vpass=1:vqscale=5:vcodec=snow:vstrict=-2:pred=0:cmp=1:subcmp=1:mbcmp=1:qpel -o snow.avi
mencoder.exe qcif.avi -ovc lavc -lavcopts vpass=3:vbitrate=500:vcodec=snow:vstrict=-2:pred=0:cmp=1:subcmp=1:mbcmp=1:qpel -o snow.avi
mencoder.exe qcif.avi -ovc lavc -lavcopts vpass=3:vbitrate=500:vcodec=snow:vstrict=-2:pred=0:cmp=1:subcmp=1:mbcmp=1:qpel -o snow.avi

Bugs observed:

At frame ~100 there are vivid stripes in the chroma in the bottom right of the video as the camera rotates.
At frame ~2550 as the camera pans up from the tractor the 'new' sections of the field have a green tint until the next keyframe.