He was playing in a comedy called "The Mating Game". Sid came on stage said a line, sat on the sofa but did not reply to the next cue. Olga Lowe who was sitting next to him thought it was a gag (for which Sid James was renowned) and started to ad lib, but soon realised that something was amiss. The curtain was brought down and the audience asked 'Is there a doctor in the house?' The audience also thinking it part of the plot greeted this with laughter and the question had to be repeated in earnest. Eventually a doctor responded, called an ambulance and Sid was then found to be dead from a heart attack.But that may not be the end of the story. To quote Wikipedia:
Later it was rumoured that his ghost was in the dressing room he occupied on the night of his death; after one experience during a gig there, the comedian Les Dawson refused to play the venue again.