Because of the difficulties getting the acts to the farm early enough, Richie Havens was asked to keep playing longer than scheduled. Running out of material, he pulls up a classic spiritual "Motherless Child," adds some improvised lyrics, and belts out "Freedom," one of the defining songs of the festival.
On the other hand, there's the scene with The Who's performance where the building music of "We're Not Going to Take it" coincides with sunrise, which proves so awesome that The Who arranged special lighting for future concerts to recreate the effect.
The nuns flashing the peace sign.
The Port-O-San maintenance guy, keeping them clean for 500,000 people.