We were playing a local late night club once and I came down with laryngitis. I was singing about 80% of the songs so this was a problem. Other members of the band started singing a few songs that they didn't usually do and really didn't know. We even got some friends out of the crowd to sing what they could, but it was still going to be a long night for us. Finally we got ahold of a singer from another band who did a lot of the same songs as us who said he'd come bale us out, but it would be an hour or so before he could get there. Well, the show had to go on, so I started a simple I, IV, V blues and the rest of the band wondered what was going on since no one sang a blues song, but they kicked right in on it. After jamming around a little bit, I walked up to my mic and in a coarse whisper (the most I could muster), I started singing:
"I got the laryngitis blues, Oh, my voice is gone tonight.
I got the laryngitis blues, Oh, my voice is gone tonight.
But my guitar wants to sing, I hope it makes ya feel alright."
Then I did an extended guitar solo, made up another verse, had the bassist do a solo, made up another verse, another guitar solo...
Anyway, it had to be the longest blues song in history, but it filled up the time until the other singer arrived and we actually got more compliments on that song than any of the others we played that night.