Avenged Sevenfold bassist Johnny Christ is clearly a man of method when it comes to pranks. Dude worked with the band's soundman for a show in Helsinki to make guitarist Synyster Gates's backing vocals sound like a chipmunk, though only to him and to the band.
Of course Gates has o idea what's going on and thinks the whole crowd can hear him sound ridiculous, making for what I'm sure was a slightly confused crowd in Helsinki.