Has a headline act ever been booed off?

would boo cake

cheap zing, sorry, haven't actually heard cake for a decade

Who has?

they weren't the headlining act, but boy did Sunny Day Real Estate get booed heavily when they opened for Bad Religion. they took it in stride though...after all, they are emo

better zing (xp)

Metallica was booed off on Dynamo Open Air 1996 when they failed to show up and thought it a good idea to do a live-via-satellite-over-the-PA interview instead where they jokingly talked about about-to-be-released Load and played two-minute snippets of 2x4 and The House Jack Built. 90.000 metalheads waiting in front of the stage almost went on a rampage when it dawned on them that this was not a joke or an intro for the billed headline act. Things could've gotten really ugly if Venom hadn't taken over and completely killed it. That moment there they lost their remaining Dutch, German and Belgian fans.

which moment, the moment where they failed to show, or the moment where they played the audience Load material?

Closest I ever saw was Smashing Pumpkins at Lollapalooza in 1994 or so. Billy Corgan went on a long tyrade about how he's the "voice of a generation, a generation of shit" and basically sounded like a brat. Then he stormed off and the crowd started a chant of "fuck you" he came back and apologized, saying "You meant fuck you in a good way, right?" What a loser.

Bob Dylan must have been some sort of headline act at Newport, wasn't he?