An engine that overheats and causes a car to catch on fire is an especially good example of how one problem can lead to another. A car's engine probably won't overheat enough to simply burst into flames all on its own. But what can happen (and pretty easily, by the way), is an engine can overheat and make the internal fluids, like oil and coolant, rise to dangerous temperatures and begin to spill out of their designated areas of circulation. When that happens, they drip, drizzle and spurt throughout the engine bay and onto the exhaust system, landing on other hot parts, where they can easily ignite and spread.
In some cases, like the late-2012 recall of about 90,000 Ford cars equipped with a specific EcoBoost powertrain, an engine that overheats is sometimes a design flaw that's fixable with a software update -- modifying the car's computer to help keep engine temperatures at a safer (lower temperature) threshold. Generally, though, an overheating engine requires mechanical attention. There's often a leaky seal or gasket, or the radiator isn't working properly, or any number of other things. If your car's engine is constantly overheating ... well, that's not a symptom to ignore.