Worst instance ever was starting A Message To You Rudy. Or rather not starting it. Specials trib, early soundcheck, long afternoon drinking.
Stepped to the front of the stage, made 'em wait for a bit then thought to myself - I have no idea what this song even sounds like.
Mortifying.
Nowadays it still happens occasionally but two significant factors have changed. One I'm sober. Two I realise I'm a musician not a heart surgeon. Nobody dies if I fluff my lines. So I just ask the rest of the band to hum or play it to me.
We were doing a Bruno Mars song which I had successfully busked with chord sheets until this particular gig. The guitarist told me I should start it.Byt there's no bass on the intro I argued. BL agreed with him and sure enough when we reached the song I had no idea. Couldn't bring it to mind. After a few fumbled attempts guitar wandered over and asked - Do you actually know this song?
I was bang to rights. Had to admit that no I didn't. Apart from when we gigged it I never heard of it.