$ ffmpeg -video_size 1024x768 -framerate 25 -f x11grab -i :0.0+100,200 output.flv
ffmpeg version 0.8.13-6:0.8.13-0ubuntu0.13.10.1, Copyright (c) 2000-2014 the Libav developers
  built on Jul 15 2014 13:48:35 with gcc 4.8.1
*** THIS PROGRAM IS DEPRECATED ***
This program is only provided for compatibility and will be removed in a future release. Please use avconv instead.
[x11grab @ 0x14e5080] device: :0.0+100,200 -> display: :0.0 x: 100 y: 200 width: 1024 height: 768
[x11grab @ 0x14e5080] shared memory extension  found
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  130 (MIT-SHM)
  Minor opcode of failed request:  4 (X_ShmGetImage)
  Serial number of failed request:  11
  Current serial number in output stream:  11