[branden@localhost ~]$ ffmpeg -i sample.avi -acodec copy -vcodec copy sample.mkv
ffmpeg version git-2012-01-09-741aca7 Copyright (c) 2000-2012 the FFmpeg developers
  built on Jan  8 2012 16:56:15 with gcc 4.6.2 20111125 (prerelease)
  configuration: --prefix=/usr --enable-gpl --enable-libfaac --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-nonfree --enable-postproc --enable-x11grab
  libavutil      51. 34.100 / 51. 34.100
  libavcodec     53. 54.100 / 53. 54.100
  libavformat    53. 29.100 / 53. 29.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 58.100 /  2. 58.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
  libpostproc    51.  2.100 / 51.  2.100
Input #0, avi, from 'sample.avi':
  Duration: 02:06:50.30, start: 0.000000, bitrate: 1654 kb/s
    Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 720x304 [SAR 1:1 DAR 45:19], 23.98 tbr, 23.98 tbn, 23.98 tbc
    Stream #0:1: Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, 5.1(side), s16, 384 kb/s
Output #0, matroska, to 'sample.mkv':
    Stream #0:0: Video: mpeg4 (XVID / 0x44495658), yuv420p, 720x304 [SAR 1:1 DAR 45:19], q=2-31, 1k tbn, 23.98 tbc
    Stream #0:1: Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, 5.1(side), 384 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
[matroska @ 0x352cf20] Can't write packet with unknown timestamp
av_interleaved_write_frame(): Invalid argument
[branden@localhost ~]$