Sour grapes on Monterey Pop?
I've read some speculation such as the following:
Frank Zappa, was initially booked to perform with his groundbreaking group, The Mothers of Invention. But Zappa reportedly refused to appear alongside the festival’s many San Francisco bands, who he deemed inferior.(emphasis & links added)
I've seen this is a few places, but none of the sources appear reliable. This is the best source I could find, but author Varga mentions that this is just "reportedly". This sure sounds like sour grapes, so we should wait for a better source (if any) before putting this on the page.