Mixed content actually happens quite a bit, at least in NTSC land. Anime often has mixed telecined (24fps progressive content broadcast at 30fps interlaced which can be inverse telecined to re-create the original 24fps progressive frames) and genuine 30fps interlaced parts. Extras on DVDs that were aired on television as promo pieces can have parts that are 30fps interlaced and parts that are really 24fps film telecined to 30fps for broadcast. TV shows from the last 20 years or so, especially sci-fi shows with CGI, are often mixed telecined content and 30fps bits (the CGI effects parts).
You can just deinterlace all of it willy-nilly, but quality deinterlacing is tough for older computer to do real-time, and then even top quality deinterlacing of a video that is actually telecined content loses the quality you'd get by properly inverse telecining it back to progressive film. That's why DVDs usually have stream flags to tell a flag-aware player what the content of the segment at hand is. But flags can be wrong, too, and there are other bizarre cadences besides just 24fps interlaced, 24/30 fps telecined and 30fps interlaced (especially encountered in badly mastered anime), so the best players do cadence detection to try to tell what the content really is regardless of what the basic stream flags indicate.
Anyway, that's what I was asking about.
Thanks for suggesting VLC, it seems to be flag responsive to some degree.