Factual error: As Eliott walks across the hillside where the concert is taking place, you can see nylon dome and bivouac style tents. Tents made with metal poles and nylons didn't come to market til the 70s. (01:23:05)
Factual error: At the end of the movie, the Boy Scouts who are cleaning up the Woodstock site are wearing knee socks with red trim, which was not part of the official uniform until much later. In 1969 the knee socks were solid green. (I know because I was working at a camp while Woodstock was happening.)