Okay, not a car fire, but in the worst extreme it could have been.
Important lesson 1: If you top up the coolant reservoir in an overheating car, the car (at least my Miata) has to cool down before it will draw the coolant from reservoir into radiator.
When I was getting the Miata back on the road after the Explorer was killed by a truck (nobody was hurt; let’s get that clear from the start, but the insurance, given the Explorer’s age, totaled it out), I had a couple of more serious problems to deal with (brakes!). As a result checking coolant level fell through the cracks. While out doing some shopping today, I noticed engine temp going up past the top end of the scale. Eep. (Important lesson 2: Pay attention to your gauges).
So I stopped and bought some coolant (gas stations and parts stores are all over the place to it was easy to find). Reservoir was essentially dry so I filled and started out again.
Engine temp continued to rise. Eep2.
At my next stop, I popped the hood and checked the coolant reservoir again. Still full. Level not a millimetre lower than it was when I’d filled it.
“Oh,” thinks I, “this is not good. Not good at all.”
I inspected the hose between reservoir and radiator thinking maybe there’s damage or a leak causing the coolant to not be picked up. Nope. Nothing wrong I could see there. I popped open the radiator cap and…no coolant visible. So, I poured more coolant into the radiator. Only, it was still hot so I got some bubbling and mini-geysering. It was a bit of a challenge to get the cap back on without getting burned/scalded but I managed. Started up the engine again and, temperature promptly fell back into normal range.
When I got home, I parked and let the engine cool for a bit. Then checked the coolant level in the reservoir. About half full, so coolant was drawn from reservoir back into the engine. I topped it up again with the bottle of pre-diluted coolant I had in the garage.
And that, boys and girls, is why you always, always check your fluid levels. Having those checks “fall through the cracks” can lead to bad things indeed.