$ /opt/bin/ffmpeg -y -i the.mkv -vn -an -codec:s dvd_subtitle the.sub
ffmpeg version git-2014-08-04-4e855c1 Copyright (c) 2000-2014 the FFmpeg developers
  built on Aug  4 2014 19:42:40 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
  configuration: --prefix=/mnt/1/src/ffmpeg/build --extra-cflags=-I/mnt/1/src/ffmpeg/build/include --extra-ldflags=-L/mnt/1/src/ffmpeg/build/lib --bindir=/opt/bin --extra-libs=-ldl --enable-gpl --enable-libass --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-nonfree --enable-x11grab --enable-libxvid
  libavutil      52. 95.101 / 52. 95.101
  libavcodec     55. 72.101 / 55. 72.101
  libavformat    55. 51.101 / 55. 51.101
  libavdevice    55. 13.102 / 55. 13.102
  libavfilter     4. 11.102 /  4. 11.102
  libswscale      2.  6.100 /  2.  6.100
  libswresample   0. 19.100 /  0. 19.100
  libpostproc    52.  3.100 / 52.  3.100
[mpeg4 @ 0x2e9ef00] Failed to parse extradata
Input #0, matroska,webm, from 'the.mkv':
  Metadata:
    ENCODER         : Lavf55.51.101
  Duration: 00:00:39.88, start: 0.000000, bitrate: 284 kb/s
    Stream #0:0: Video: mpeg4 (Simple Profile), yuv420p, 240x320 [SAR 16:9 DAR 4:3], SAR 137:77 DAR 411:308, 25 fps, 25 tbr, 1k tbn, 25 tbc (default)
    Metadata:
      ENCODER         : Lavc55.72.101 mpeg4
    Stream #0:1: Subtitle: dvd_subtitle (default)
[microdvd @ 0x2eae8e0] Exactly one MicroDVD stream is needed.
Output #0, microdvd, to 'the.sub':
  Metadata:
    encoder         : Lavf55.51.101
    Stream #0:0: Subtitle: dvd_subtitle (dvdsub) (default)
    Metadata:
      encoder         : Lavc55.72.101 dvdsub
Stream mapping:
  Stream #0:1 -> #0:0 (dvd_subtitle (dvdsub) -> dvd_subtitle (dvdsub))
Could not write header for output file #0 (incorrect codec parameters ?): Operation not permitted

$ /opt/bin/ffmpeg -y -i the.mkv -vn -an -codec:s copy the.sub
ffmpeg version git-2014-08-04-4e855c1 Copyright (c) 2000-2014 the FFmpeg developers
  built on Aug  4 2014 19:42:40 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
  configuration: --prefix=/mnt/1/src/ffmpeg/build --extra-cflags=-I/mnt/1/src/ffmpeg/build/include --extra-ldflags=-L/mnt/1/src/ffmpeg/build/lib --bindir=/opt/bin --extra-libs=-ldl --enable-gpl --enable-libass --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-nonfree --enable-x11grab --enable-libxvid
  libavutil      52. 95.101 / 52. 95.101
  libavcodec     55. 72.101 / 55. 72.101
  libavformat    55. 51.101 / 55. 51.101
  libavdevice    55. 13.102 / 55. 13.102
  libavfilter     4. 11.102 /  4. 11.102
  libswscale      2.  6.100 /  2.  6.100
  libswresample   0. 19.100 /  0. 19.100
  libpostproc    52.  3.100 / 52.  3.100
[mpeg4 @ 0x2db8f00] Failed to parse extradata
Input #0, matroska,webm, from 'the.mkv':
  Metadata:
    ENCODER         : Lavf55.51.101
  Duration: 00:00:39.88, start: 0.000000, bitrate: 284 kb/s
    Stream #0:0: Video: mpeg4 (Simple Profile), yuv420p, 240x320 [SAR 16:9 DAR 4:3], SAR 137:77 DAR 411:308, 25 fps, 25 tbr, 1k tbn, 25 tbc (default)
    Metadata:
      ENCODER         : Lavc55.72.101 mpeg4
    Stream #0:1: Subtitle: dvd_subtitle (default)
[microdvd @ 0x2dc88e0] Exactly one MicroDVD stream is needed.
Output #0, microdvd, to 'the.sub':
  Metadata:
    encoder         : Lavf55.51.101
    Stream #0:0: Subtitle: dvd_subtitle (default)
Stream mapping:
  Stream #0:1 -> #0:0 (copy)
Could not write header for output file #0 (incorrect codec parameters ?): Operation not permitted