Comment 21 for bug 305286

I've tried this guide
http://verb3k.wordpress.com/2010/01/26/how-to-do-proper-screencasts-on-linux/

this gave the same problem:
ffmpeg -f alsa -i pulse -f x11grab -r 30 -s 1024x768 -i :0.0 -acodec pcm_s16le -vcodec libx264 -vpre lossless_ultrafast -threads 0 output.mkv

but converting to avi solved it:
ffmpeg -i output.mkv output.avi