I'm converting MKV files to MP4 with ffmpeg, and when I try to open some (but not all!) of the resulting files, QuickTime Player (version 10.4 on macOS 10.13.6) produces an error dialog with the message:

The document “….mp4” could not be opened. An unknown error occurred (-50)

What is this "unknown error (-50)" that is preventing my MP4's from being opened? How can I get more details about the error so that I know how to fix it?

  • Can you edit what version of ffmpeg creates the broken files and perhaps what commands are passed and ideally what is in the MKV container into the question?
    – bmike
    Aug 17, 2018 at 21:19
  • @bmike: ffmpeg isn't the one producing the error; QuickTime is. I want to know how to get more detail from QuickTime about the error so that I know what to fix in ffmpeg. Running ffmpeg is a separate question from what I'm asking here.
    – jwodder
    Aug 17, 2018 at 21:21

1 Answer 1


That looks like a classic Mac OS error code. After a bit of googling I came up with:

-50 | paramErr | Error in user parameter list

Which indicates there is an bug in the program. You should contact the developer about the issue.

It is possible that there is a command line flag that is incorrect and causing the issue.

Reference: http://support.apple.com/kb/ht1618

  • Why would a system error from Mac OS 9 be showing up in QuickTime under High Sierra?
    – jwodder
    Aug 17, 2018 at 19:36
  • @jwodder Same people designed the error codes and they are more stable than most documentation in my experience. The include files that make the program would have to be sourced to know for sure. +1 answer in my book. Why would you think quicktime didn't process video files correctly and think it's not a problem with the file?
    – bmike
    Aug 17, 2018 at 21:24

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