Electrical smoke
One of the main things to consider is how quickly a situation can develop.
Running start checks on a C130 the Flight Eng said "Look at that" looked up to see small wisps of smoke, almost like small seahorse shape descending from overhead panel. As we went to turn off power it quickly developed into dense smoke and filled the flight deck.
All from a minor fault behind the panel and a plastic piece smouldering. Luckily on the ground.