[FFmpeg-user] transcoding belches out "Past duration 0.939156 too large"

[jd1008]

using 3.0.2-static:

ffmpeg -i vid_1.mkv -vb 4000k -s 1920x1080 -vf hflip,scale=1920:1080 -y 
vid_1-hflip.mkv

is belching out lots of
Past duration 0.939156 too large

Any ideas why?