In the 80s and 90s, regulation increased to prohibit smoking in passenger planes. Was smoking ever identified as the cause for serious accident in flight (for example the cabin caught fire as a result of a stray cigarette on the carpeting or seats)?
24 December 1982; CAAC Ilyushin 18B; Canton (Guangzhou), China: The aircraft was on a scheduled domestic flight when an onboard fire broke out as a result of a carelessly discarded cigarette. The aircraft landed and the passengers and crew evacuated, but 25 of the 50 passengers were killed. All 11 crew members survived. source
The cause of the accident of a DH 86 Express on 2 October 1935 is not confirmed, but likely caused by a cigarette, albeit indirectly. From the Wikipedia article on the DH 86:
On 2 October 1935 Holyman's VH-URT Loina was also lost in Bass Strait, again with no survivors. This time a significant amount of wreckage was recovered from the sea and from beaches on Flinders Island. Investigation of the wreckage revealed a section of charred carpet on a piece of cabin flooring from just ahead of the lavatory door. It was thought possible that a small fire from a dropped cigarette had led to someone running aft suddenly to stamp it out – the sort of sudden change in weight distribution that could set up a fatal loss of directional control while the aircraft was on a low-speed landing approach.