What does the FFmpeg warning “Missing picture in access unit with size x” mean

I’ve recently updated to the most recent version of ffmpeg and now every time I edit a flv, I always get endless warnings:

[Null @ 002de180] Missing Missing picture in access unit with size
xxxx (size varies per file)

[Null @ 002de180] AVC: nal size xxxx (size also varies per
file)

…..

…

I’ve checked other versions too and it seems like any version above 2.1.4 will start giving these warnings.

The command I'm using is ffmpeg.exe -t 0:03:00 -y -i "C:\test.flv" -vcodec copy -acodec copy "C:\Desktop\newvid.flv" to cut sections of a video.

Even with these warnings, the output file plays fine and the video seems to be in sync. The file size is also the same as if I edit the vid with ffmpeg 2.1.4. But the versions with these warnings take much longer to encode. Any idea what is happening or what these warnings mean?