This is also happening to me on my Vizio M65-D0 with SmartCast, MKV and MP4 so far that I can confirm (note: not all MKV and MP4 files are having this happen for me, only some). First time it happened was a couple of weeks ago, before that I could watch those same videos just fine. I can adjust the aspect ratio on the TV to fill it out vertically, but then it pushes the sides of the video off the screen. Also, it only does it sometimes for me, other times those same videos will play like normal.