Datamoshing is a form of glitch art which occurs when ‘the I-frames or key-frames of a temporally compressed video are removed, causing frames from different video sequences to bleed together‘.
They guy behind these videos seems to have popularised the technique with a Chairlift music video for Evident Utensil, but Sony would rather you didn’t see it.
(Shh, it’s on Metacafe.)
AviGlitch is a Ruby library claims to be the easiest way to datamosh a video file. (In this case, ‘easy’ seems to assume you understand the first thing about Ruby.)