When they are at the football game, close up shots of the band were done earlier during the week and the field was dry. Long shots were done on an actual game on a rainy Saturday, so there are two colors of Astroturf being shown back and forth.
When Peter Fonda goes on stage in the beginning he wears a right handed guitar but plays it left handed. This is typically done if the guitar is restrung for left handed play, but this one is strung for right handed play; the heavier strings are now on the bottom.