title: Strange "pause" in video after concatenation of two mp4 videos
body: <p>I am concatenating two mp4 videos. The problem is that first video (intro.mp4) lasts 5 seconds, second video (output.mp4) lasts 2 seconds and video created by concatenating them lasts 9 seconds (and should last 5+2 = 7 seconds). In final.mp4 video the last frame from first video (intro.mp4) i shown for additional 2 seconds before second video (output.mp4) is played. It looks like a lag when watching video. What I am doing wrong?</p>
  
  <p>list.txt:</p>
  
  <pre><code>file 'data/intro.mp4'
  file 'output.mp4'
  </code></pre>
  
  <p>command:</p>
  
  <pre><code>./bin/ffmpeg -f concat -i list.txt -c copy final.mp4
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>ffmpeg version 2.7.2 Copyright (c) 2000-2015 the FFmpeg developers
    built with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
    configuration: --prefix=/Volumes/Ramdisk/sw --enable-gpl --enable-pthreads --enable-version3 --enable-libspeex --enable-libvpx --disable-decoder=libvpx --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-avfilter --enable-libopencore_amrwb --enable-libopencore_amrnb --enable-filters --enable-libgsm --enable-libvidstab --enable-libx265 --disable-doc --arch=x86_64 --enable-runtime-cpudetect
    libavutil      54. 27.100 / 54. 27.100
    libavcodec     56. 41.100 / 56. 41.100
    libavformat    56. 36.100 / 56. 36.100
    libavdevice    56.  4.100 / 56.  4.100
    libavfilter     5. 16.101 /  5. 16.101
    libswscale      3.  1.101 /  3.  1.101
    libswresample   1.  2.100 /  1.  2.100
    libpostproc    53.  3.100 / 53.  3.100
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f8a6a80e600] Auto-inserting h264_mp4toannexb bitstream filter
  Input #0, concat, from 'list.txt':
    Duration: N/A, start: 0.000000, bitrate: 1296 kb/s
      Stream #0:0: Video: h264 (High) (avc1 / 0x31637661), yuv420p, 960x540, 1163 kb/s, 23.98 fps, 23.98 tbr, 11988 tbn, 47.95 tbc
      Stream #0:1: Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 133 kb/s
  [mp4 @ 0x7f8a6c006a00] Codec for stream 0 does not use global headers but container format requires global headers
  [mp4 @ 0x7f8a6c006a00] Codec for stream 1 does not use global headers but container format requires global headers
  Output #0, mp4, to 'final.mp4':
    Metadata:
      encoder         : Lavf56.36.100
      Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 960x540, q=2-31, 1163 kb/s, 23.98 fps, 23.98 tbr, 11988 tbn, 11988 tbc
      Stream #0:1: Audio: aac ([64][0][0][0] / 0x0040), 48000 Hz, stereo, 133 kb/s
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (copy)
    Stream #0:1 -&gt; #0:1 (copy)
  Press [q] to stop, [?] for help
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f8a6a80e600] Auto-inserting h264_mp4toannexb bitstream filter
  frame=  258 fps=0.0 q=-1.0 Lsize=     997kB time=00:00:09.84 bitrate= 829.5kbits/s    
  video:897kB audio:93kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.634306%
  </code></pre>