At the part where Marty is filming Doc explain how the time machine works at the Twin Pines Mall, Doc says "Nevermind that now, never mind that now," and Marty doesn't catch it on film, infact the camera is pointed towards the ground. But when the 1955 Doc is watching the tape, it shows 1985 Doc saying that line.
If you watch this you'll notice that isn't the only difference between what Marty records and what they see in 1955. The part where he films the car driving around then travelling through time goes on for ages but in 1955 they 'fast-forward' the tape and this part appears as just a flash which would have been just a few seconds of film if viewed at normal speed. Oh well, maybe Marty's time travelling changed the future so he recorded different things? :)
This is a very good slip up in my opinion. Kudos on noticing it.
I also noticed that on that video, when Doc says about needing a nuclear
reaction to generate the 1.21 gigawatts, he starts to explain something
about the flux capacitor, but you never hear completely what it was he said.
This bit is clearly not in the Parking Lot sequence, as Marty straight
away asks Doc if he ripped the plutonium off. Doc never mentions anything
about the flux capacitor
Here's a thought...Perhaps Marty's time traveling altered the future (just a little) and affected what he recorded Doc doing, and the videotape changed, like the photograph of Marty and his siblings.
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