It could be a bad encoding for the file. Heavy (obvious) pixelating is the digital version of static, so while it might be a very rare thing, it may be that the data wasn't read off the drive properly. I don't know how to pinpoint if it was the TV or drive definitively.
What happens on the other sources with HD content? Do those also pixelate like the media on the drive?
I recently bought a UN40B7000WF. The system worked great for the first 2 weeks. Then suddenly in the middle of a movie the picture turned heavily pixelated. Note that the source was a USB key so I have ruled out quality issues with the source. Even the built-in Content pictures appear the same.
I did a reset procedure as suggest by the Samsung Tech support but no luck.
Any idesa on what this could be ?