Car wont catch on fire. Eventually your engine could do a few things. 1. Seize/stop working/break The coolant strips away the lube abilities in the oil and the engine, unless there is oil involved there can be metal on metal. You don't want this. Coolant in the oil promotes this.
2. Your headgasket will leak so much that coolant will fill the engine, it will overheat and you will lose power and not be able to drive it.