The drum kit was inside a giant seashell, which would open after the show started, revealing [drummer] Alan [White] doing his stuff. However, one night when the curtain went up the gearing jammed and he was trapped inside. The problem was, it was a sealed unit, so Alan quickly began running out of air.
As this was live on stage in front of thousands of people, Alan, the consummate professional, continued playing. Meanwhile the roadies began trying to smash the pod open, staying out of the line of sight of the crowd so no one noticed.
Before long, they had to start pumping oxygen in until eventually, somehow, they prised the wretched thing open with pickaxes.
By now the audience must have noticed the rescue effort because as the pod sprang open a huge cheer went up, and Alan stumbled out gasping for breath.