gistfile1.txt

ffmpeg -i Safari\ Browsing
ffmpeg version 2.6.3 Copyright (c) 2000-2015 the FFmpeg developers
  built with Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/2.6.3 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libfreetype --enable-libvorbis --enable-libvpx --enable-libass --enable-ffplay --enable-libfdk-aac --enable-libopus --enable-libquvi --enable-libx265 --enable-nonfree --enable-vda
  libavutil      54. 20.100 / 54. 20.100
  libavcodec     56. 26.100 / 56. 26.100
  libavformat    56. 25.101 / 56. 25.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 11.102 /  5. 11.102
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, matroska,webm, from 'Safari Browsing':
  Metadata:
    encoder         : QTmuxingAppLibWebM-0.0.1
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0(eng): Video: vp8, yuv420p, 1958x1514, SAR 1:1 DAR 979:757, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
    Stream #0:1(eng): Audio: vorbis, 44100 Hz, mono, fltp (default)
At least one output file must be specified

gistfile2.txt

ffmpeg -i /var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmpQljqJo.avi -i /var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmp_6oreh.avi -c:v huffyuv -preset veryslow -y -filter_complex [0:1][1:1]amix=inputs=2:duration=first;overlay=x=195.49782334:y=0 /var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmp1btwYj.avi
ffmpeg version 2.6.3 Copyright (c) 2000-2015 the FFmpeg developers
  built with Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/2.6.3 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libfreetype --enable-libvorbis --enable-libvpx --enable-libass --enable-ffplay --enable-libfdk-aac --enable-libopus --enable-libquvi --enable-libx265 --enable-nonfree --enable-vda
  libavutil      54. 20.100 / 54. 20.100
  libavcodec     56. 26.100 / 56. 26.100
  libavformat    56. 25.101 / 56. 25.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 11.102 /  5. 11.102
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, avi, from '/var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmpQljqJo.avi':
  Metadata:
    encoder         : Lavf56.25.101
  Duration: 00:00:23.36, start: 0.000000, bitrate: 92167 kb/s
    Stream #0:0: Video: huffyuv (HFYU / 0x55594648), yuv422p, 1280x720, 92318 kb/s, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 25 tbn, 25 tbc
Input #1, avi, from '/var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmp_6oreh.avi':
  Metadata:
    encoder         : Lavf56.25.101
  Duration: 00:00:23.07, start: 0.000000, bitrate: 85719 kb/s
    Stream #1:0: Video: huffyuv (HFYU / 0x55594648), yuv422p, 878x700, 85762 kb/s, SAR 342650:332323 DAR 979:757, 30 fps, 30 tbr, 30 tbn, 30 tbc
    Stream #1:1: Audio: mp3 (U[0][0][0] / 0x0055), 44100 Hz, mono, s16p, 64 kb/s
Stream specifier ':1' in filtergraph description [0:1][1:1]amix=inputs=2:duration=first;overlay=x=195.49782334:y=0 matches no streams.

for legability purposes the 2nd command is

ffmpeg -i /var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmpQljqJo.avi 
-i /var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmp_6oreh.avi 
-c:v huffyuv -preset veryslow -y 
-filter_complex [0:1][1:1]amix=inputs=2:duration=first;overlay=x=195.49782334:y=0 
/var/folders/t6/2vcrxfcd0953m57lybpmh8xr0000gp/T/tmp1btwYj.avi