I was there! The place was so mobbed it was a fire risk, so they used various tactics to get rid of people (none worked) until they eventually said they weren't coming. The place cleared, leaving around 50 people, and Kurt and Dave arrived playing 6-7 acoustic numbers. Amazing!
I asked Kurt if he'd sorted out the set list for the next night in Newcastle, he said "More or less, why?" I said "I was disappointed you didn't play Negative Creep on Friday, any chance you could play it tomorrow?" He said, "I don't see why not." ... and the did!! :)

On the 1st of December 1991, playing under the name Teen Spirit, Nirvana played an unannounced acoustic set to around 20 people. Kurt Cobain played acoustic guitar and Dave Grohl, acoustic bass.