This is a serious post about a serious issue -- restarted due to distracting responses. The best way to see and replicate the problem is to run this script (which creates three AC3s and a FOO.TXT on c:\): ffmpeg -y -f lavfi -i anullsrc=channel_layout=2:sample_rate=48000 -t 240.0 -c ac3 "c:\file1.ac3" ffmpeg -y -f lavfi -i anullsrc=channel_layout=2:sample_rate=48000 -t 240.0 -c ac3 "c:\file2.ac3" echo ffconcat version 1.0>c:\concat.txt echo file 'file1.ac3'>>c:\concat.txt echo file 'file2.ac3'>>c:\concat.txt ffmpeg -y -safe 0 -i "c:\concat.txt" "c:\concat.ac3" echo this is foo>c:\FOO.TXT On my computer, '-t 240.0' created silent AC3s that were long-large enough to provoke the issue -- on your system, '-t' may need to be bigger. In the terminal output shown below, note these lines: <quote> Enter command: <target>|all <time>|-1 <command>[ <argument>] Parse error, at least 3 arguments were expected, only 1 given in string 'ho this is foo>c:\foo.txt' </quote> Egad! That should not be happening. To me, it indicates that after a certain length of execution time, FFmpeg goes into some sort of command mode during which it fetches the 'echo' string and responds to it as though it was a command to FFmpeg. WTF? If FFmpeg does have some sort of automatic command mode that it monitors, isn't that asking for trouble? Certainly I consider it a bug (or design flaw). C:\Windows\System32>ffmpeg -y -f lavfi -i anullsrc=channel_layout=2:sample_rate=48000 -t 240.0 -c ac3 "c:\file1.ac3" ffmpeg version 2025-07-23-git-829680f96a-full_build-www.gyan.dev Copyright (c) 2000-2025 the FFmpeg developers built with gcc 15.1.0 (Rev6, Built by MSYS2 project) configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-lc ms2 --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enabl e-avisynth --enable-libbluray --enable-libcaca --enable-libdvdnav --enable-libdvdread --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enab le-libdavs2 --enable-libopenjpeg --enable-libquirc --enable-libuavs3d --enable-libxevd --enable-libzvbi --enable-liboapv --enable-libqrencode --enable-librav1e --enable -libsvtav1 --enable-libvvenc --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxeve --enable-libxvid --enable-libaom --enable-libjxl --en able-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidst ab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va --enable-ffnvcodec --enable-libvpl --enable-nvdec --enable-nvenc --enable-vaapi --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-openal --enable-libgme -- enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc -- enable-libcodec2 --enable-libilbc --enable-libgsm --enable-liblc3 --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --ena ble-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint libavutil 60. 6.100 / 60. 6.100 libavcodec 62. 8.100 / 62. 8.100 libavformat 62. 1.103 / 62. 1.103 libavdevice 62. 0.100 / 62. 0.100 libavfilter 11. 3.100 / 11. 3.100 libswscale 9. 0.100 / 9. 0.100 libswresample 6. 0.100 / 6. 0.100 [out @ 0000000001cabf40] The "sample_fmts" option is deprecated: set the supported sample formats [out @ 0000000001cabf40] The "all_channel_counts" option is deprecated: accept all channel counts Input #0, lavfi, from 'anullsrc=channel_layout=2:sample_rate=48000': Duration: N/A, start: 0.000000, bitrate: 384 kb/s Stream #0:0: Audio: pcm_u8, 48000 Hz, 1 channels (FR), u8, 384 kb/s Stream mapping: Stream #0:0 -> #0:0 (pcm_u8 (native) -> ac3 (native)) Press [q] to stop, [?] for help Output #0, ac3, to 'c:\file1.ac3': Metadata: encoder : Lavf62.1.103 Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s Metadata: encoder : Lavc62.8.100 ac3 [out#0/ac3 @ 0000000001cac040] video:0KiB audio:5625KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 0.000000% size= 5625KiB time=00:03:59.99 bitrate= 192.0kbits/s speed= 329x elapsed=0:00:00.72 C:\Windows\System32>fmpeg -y -f lavfi -i anullsrc=channel_layout=2:sample_rate=48000 -t 240.0 -c ac3 "c:\file2.ac3" 'fmpeg' is not recognized as an internal or external command, operable program or batch file. C:\Windows\System32>echo ffconcat version 1.0>c:\concat.txt C:\Windows\System32>echo file 'file1.ac3'>>c:\concat.txt C:\Windows\System32>echo file 'file2.ac3'>>c:\concat.txt C:\Windows\System32>ffmpeg -y -safe 0 -i "c:\concat.txt" "c:\concat.ac3" ffmpeg version 2025-07-23-git-829680f96a-full_build-www.gyan.dev Copyright (c) 2000-2025 the FFmpeg developers built with gcc 15.1.0 (Rev6, Built by MSYS2 project) configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-lc ms2 --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enabl e-avisynth --enable-libbluray --enable-libcaca --enable-libdvdnav --enable-libdvdread --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enab le-libdavs2 --enable-libopenjpeg --enable-libquirc --enable-libuavs3d --enable-libxevd --enable-libzvbi --enable-liboapv --enable-libqrencode --enable-librav1e --enable -libsvtav1 --enable-libvvenc --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxeve --enable-libxvid --enable-libaom --enable-libjxl --en able-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidst ab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va --enable-ffnvcodec --enable-libvpl --enable-nvdec --enable-nvenc --enable-vaapi --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-openal --enable-libgme -- enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc -- enable-libcodec2 --enable-libilbc --enable-libgsm --enable-liblc3 --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --ena ble-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint libavutil 60. 6.100 / 60. 6.100 libavcodec 62. 8.100 / 62. 8.100 libavformat 62. 1.103 / 62. 1.103 libavdevice 62. 0.100 / 62. 0.100 libavfilter 11. 3.100 / 11. 3.100 libswscale 9. 0.100 / 9. 0.100 libswresample 6. 0.100 / 6. 0.100 [ac3 @ 00000000026f2ac0] Estimating duration from bitrate, this may be inaccurate Input #0, concat, from 'c:\concat.txt': Duration: N/A, start: 0.000000, bitrate: 192 kb/s Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s Stream mapping: Stream #0:0 -> #0:0 (ac3 (native) -> ac3 (native)) Press [q] to stop, [?] for help Output #0, ac3, to 'c:\concat.ac3': Metadata: encoder : Lavf62.1.103 Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s Metadata: encoder : Lavc62.8.100 ac3 [ac3 @ 00000000026f8240] Estimating duration from bitrate, this may be inaccurate00.50 Enter command: <target>|all <time>|-1 <command>[ <argument>] Parse error, at least 3 arguments were expected, only 1 given in string 'ho this is foo>c:\foo.txt' [out#0/ac3 @ 000000000280bd00] video:0KiB audio:8438KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 0.000000% size= 8438KiB time=00:05:59.99 bitrate= 192.0kbits/s speed=300x elapsed=0:00:01.20 C:\Windows\System32>