After updating my installation of ffmpeg and ffmpeg-php on one of my web servers I noticed that images grabbed using ffmpeg-php's getFrame function spat out weird garbage, usually three corrupt looking frames across the top and static in the bottom two thirds. I tried tinkering with it for a very long time with no success until putting it off for a few months and tonight I finally found the solution: recompiling ffmpeg with the bindist use flag.

# echo "media-video/ffmpeg cpudetection bindist" >> /etc/portage/package.use; emerge ffmpeg

Still not sure exactly what the problem was but glad to have it fixed now, all those corrupt thumbnails were embarrassing :s