After a lot packages were migrated to virtual/ffmpeg /usr/bin/ffmpeg and support for ffmpeg syntax were removed from libav. Several packages are still using ffmpeg CLI directly and do not know about new avconv syntax. This problem is not detected by tinderbox, because it is not build time, but run time failure.
Now packages do not indicate anyhow if they are using libs, cli or both from ffmpeg or libav. So it is hard to find problematic packages. I checked most of suspicious packages, but could miss some.
media-video/libav-0.8.7 has /usr/bin/ffmpeg, but libav-9.* do not.