File this one under: Mind-blowing. Robert Zemeckis shot the first five weeks of Back to the Future with Eric Stoltz playing Marty McFly, before realizing he sucked and brought in Michael J. Fox. More here.
This could create a time paradox, the results of which could cause a chain reaction that would unravel the very fabric of the space time continuum, and destroy the entire universe. Granted, that's a worse case scenario. The destruction might in fact be very localized, limited to merely our own galaxy.