What is QuickTime "unknown error (-50)"? 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?
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