Never fainted on stage, but I didn't have any tea once and felt very woozy and just not focused on the gig. Ran to the shop and got loads of chocolate and crisps, survived the gig, then just lay down on the floor in the pub. Not one of my better moments.
Felt awful first time I saw Weezer too...first 'mosh pit' with a few thousand geeks and as soon as I got in the fresh air I nearly hit the deck.