British rockers The Move were scheduled to support headliners Iron Butterfly for those nights at the Fillmore, but instead dropped out and the Zeppelin signed on--much to the misgivings of IB's people, who were label mates (IB was on Atco, LZ on the more respected, big brother Atlantic imprint). By the end of Zeppelin's first set--they did two each evening, at 8 & 11:30 pm--they'd done two encores and had the crowd in such a lathered frenzy that Iron Butterfly waited a good 45 minutes before beginning their anticlimactic set. You can reviews of the show and check out some very cool memorabilia from those nights (including every page of the program everyone received those nights) here. For $3, $4 & $5 a ticket, people saw history for a bargain.
A mere four months later, May 30th 1969, Led Zeppelin returned to the Fillmore--but this time as headliners. No, Iron Butterfly wasn't reduced to being the opening act this time--that chore went to Delaney & Bonnie & Friends and jazz great Woody Herman and his orchestra. Bill Graham loved putting variety on his bills, like having Miles Davis open for The Grateful Dead, Chicago open for Zappa & The Mothers or Chuck Berry open for The Who. You can read all about those May '69 LZ shows here.
But what goes around....
On the bottom of page 20 here (you may have to scroll up a little), check out what happened later in 1969 when Led Zep's manager Peter Grant forcibly made opening band Grand Funk's manager stop their warm-up set because they were upstaging the Zeppelin. Sure, LZ could dish it out as support, but now as headliners? It wasn't long after this they decided that being the sole act on the bill was probably best--for finances AND karma.