Detuned his guitar during a performance
One famous on-stage incident happened at the Fillmore in San Francisco. During the song "Interstellar Overdrive," Syd began detuning his guitar, for no particular reason at all. Something like that might be cool if it's planned out and done deliberately, but randomly detuning your guitar with no rhyme or reason to it? That's just an a-hole move.
(AP Photo, Files, PA)