In at least a couple of scenes with Elliott walking around the side of the hotel, you can clearly see a NID or Telephone Network Box. Almost everyone has them on the side of their houses now, but these weren't in use until many years later.
When Elliot was walking past the wooden sheds in the mud, a man in air force uniform passed behind him. He had 2 F-16 model jets on his hat, and a B-1B bomber model on his chest. These were not built until the 70s/80s, and not a symbol for the Vietnam era.
One of the production staff refers to the Hog Farm commune leader Hugh Romney as "Wavy Gravy". Romney, by his own account, was not given this nickname until about a month after Woodstock at the Texas Pop Festival (supposedly by BB King).
The limousine that Michael Lang and company ride off in after their initial visit to the Teichbergs is a 1970 Cadillac, first introduced in September of 1969. The film takes place in July/August of 1969.
When Mike and Tisha first arrive at the El Monaco, Tisha is carrying her dog in a tote bag, but as they are walking around checking out the property, she's carrying the dog in her arms and the handbag is gone.
Billy talks about beating Monticello High School and catching 3 touchdown passes. Monticello High School had no football team at that time nor did it have one for decades prior to the festival. Also, if Billy was from White Lake or Bethel, he would have attended Monticello High School and would not have been playing against them.