Where were you last night?

I like this story so much I don't care if it's true or not (or that it doesn't - as far as I know - contain any Sarahs). But it's about a concert where the guest conductor didn't show up. Just before they were supposed to start, someone remembered that one of the violinists had conducted the work at college, so he was seized and put on the rostrum. Despite everything the concert went well, but the next night when he took his place back with the other violinists, the guy next to him asked, 'So where were you last night?'