Indiana Jones and the Last Crusade - Magic Petrol
Nearly half way through the movie, Indy and Elsa discover the secret tomb of Sir Richard. Indy tells us that they are wading through petrolium, and sure enough, a member of the 'brotherhood of the crucified sword' sets it alight. Before this happens, however, as Indy approaches the coffin, bits of burning rag from his torch can clearly be seen dropping into the 'petrol'. If it were petrol, the two would have been incinerated immediately, spelling the end of Dr. Jones, and indeed the movie.