http://www.stevehoffman.tv/forums/archive/index.php/t-211803-p-4.html
Somebody posted about a recollection by Mark Farner of Grand Fund Railroad...........I don't know if this is the specific instance but truth be told, if I was in that audience I'd have booed too.
'Here's Mark Farner's account below. Peter Grant did pretty much the same thing at the Bath Festival in 1970 (on a band called The Flock) so that Zep could go on as the sun was setting.
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What happened -- that story arose from an incident. We opened for Led Zeppelin at Olympia in Detroit. And the crowd was going nuts. We just broke into "Inside Looking Out" and the whole audience just (cheered) and at that point, Led Zeppelin's manager, Peter Grant, jerked the power on us. He shut the whole PA system down ... So then Terry (Knight, Grand Funk Railroad's manager at the time) comes out, they turn a microphone on, he says ... "Grand Funk is going to have to leave the stage." And the audience went boo. And Zeppelin waited an hour and a half before they went on, after we went off ... We were supposed to be on for a tour, but that was the only night we got to open for them ...