We were rehearsing in a bar in Manistee, Michigan one afternoon called The Silver Fox. At some point, our guitarist broke a string or something and needed to take a few, so that left the rest of us standing around, and the bartender yells "Hey do you guys know any Emerson, Lake & Palmer?" As it happens, we had been messing around with "Tarkus" just for the hell of it (it's not like we were ever going to play it during a set, but I had the sheet music so what the hell). So the bassist, drummer and I looked at each other and grinned, and started it up. We played maybe 12 or 16 bars of that insane 10/8 intro, then stopped and looked over at him, asked him what he thought. He was amazed. "Are you kidding?! Tarkus?! I have never heard that played in bar before!"
He thought it was so cool that he gave us a round of drinks on the house. That particular bar was actually pretty generous that way; we each got two drinks per set per night. Presumably one for us and one for a friend. Anyway, we didn't tell him that what we'd played was all we actually knew, but the bit we did play sounded pretty good.