Something that conspiracy sites do discredit their theories without opposing anything. Phrases like
Some people have also seen another film of the assassination—not yet seen by the public—which shows this stop very clearly
take away from their credibility. If no one has seen it, what is there to convince the reader that you didn't just crap it out?
Either way, I doubt it is true, if for the same reasons I doubt 9/11 was a government job. There is way too much at stake to make mistakes that could be picked out merely by watching a film frame by frame. If the government was to alter the video, you could be damn sure that it'd be perfect.