The idea that Vietnam veterans in the United States were spat on when they returned seems to have taken hold. It's mentioned in the movie First Blood ("Rambo"):

Then I come back to the world and I see all those maggots at the airport protesting me, spitting. Calling me baby killer and all that vile crap! Who are they to protest me? Who are they to protest me, huh? Who are they? Unless they've been me and been there and know what the hell they're yelling about!

And songs and in books, but there don't seem to be any first hand accounts of it.

In his book The Spitting Image Jerry Lembcke claims that it's a myth and never happened, or at least not in any confirmed incident. Furthermore it states that the peace movement were the veterans' natural allies, rather than likely to spit on them.

What is the truth here?