[FFmpeg-user] Determine Reason for "error: Error while decoding stream #0:X: Invalid data found when processing input"

[Er Win]

Hey there,

I run ffmpeg on an archive of video files and quite a bunch of them return
"Error while decoding stream #0:1: Invalid data found when processing input"

As an error, usually right at the beginning of the process.
When I play such files in several players I do not notice any problem. What does this error mean? Is stream X checked or ignored if ffmpeg gives this output?

Thank you.