Me and several friends were there for the last two nights of the '81 American Tour. One of the nights was Keith's birthday and there's a story around about someone who went backstage after the show and still has what was left of Keith's birthday cake. So cool seeing the balloons released from the ceiling slowly making their way down to the crowd and the stage. The guy running on to the stage happened so fast we didn't really know what happened.
I remember reading in an article Keith talking about the incident. It was a little over a year since John Lennon was murdered and so when you see someone rushing you on stage you would feel a bit paranoid about what could happen!