Mine would be Roll Deep in Victoria Park for Love Music Hate Racism. After enduring **** like Hard-Fi and Jay Sean i felt entitled to see someone i liked and about 3 of them came on, played about 1 and half songs and then just kinda fucked off.
I like Type O Negative, but when I saw them Peter Steele was being a tool. He would often stop singing in the middle of the song and leave his bandmates to go on without him. I don't know if he was drunk or stupid. Either way, very disappointing.
Probably Arcade Fire. Although it wasn't a disaster, it was just that they never really seemed to connect with the crowd in any way and they just didn't want to be there. And the additional percussionists were just annoying and detracted from the 'real' merabers of the band. It could have been such a mind-blowing show but it was far from it.