I once watched a buddy of mine start his gas tank on fire while using a lighter to help him see if there was any fuel in his bug. We looked at each other with retarded surprise, and he just covered the fill hole that was now on fire with his hand. Luckily we didn't explode that day!
Sometimes it looks like they "explode", but this is usually a result of fire being introduced during or before a collision that ruptures the fueltank. The expanding flaming gasoline makes such a large cloud so quickly that it's often mistaken for an explosion caused by pressure. Fire departments have neat stories.