Oops, I forgot the other possibilities:
2. There was a transfer error, resulting in ccorrupt data. Some video formats handle this very well (most mpeg codecs do), but some corrupt asf files will kick out when it hits the bad data (it is often possible to skip over these errors by simply fastfowarding past them) and avi files encoded with the divx codec will often have the video freeze while the audio keeps playing (again, this can be resolved by starting the video at a point after the error)

and 3: There is a decency chip built into many video cards today that attempts to detect pornography through excessive use of skin tones. If such skin tones are detected in a single stream of data for a long enough period, the chip will kill the video stream at the hardware level.