In some of the behind the scenes shorts Peter Jackson had to shut down production for a week or two to reset and figure out the final fight was supposed to go . He couldn't remember how the story went or how to film it.
In Iron Man, Jon Favreau and Robert Downey Jr. didn't have anything for some scenes in the script on shooting days. They just discussed what to do and winged it. Crazy way to shoot a multimillion dollar movie.