For e.g. flash encodings the videos are deinterlaced, however, this is not the case for the player-preview image. I spotted this in a case where the preview image looked really ugly. See the attached patch for a fix.
Btw. adding -deinterlace might also be necessary for profile.search-cover.http.ffmpeg.command, profile.player-slides.http.ffmpeg.command, and maybe profile.text-analysis.http.ffmpeg.command (since -deinterlace is already used for profile.video-segmentation.http.ffmpeg.command).