Actually, there can be considerable back flow from the exhaust system to the intake manifold. On most engines, the intake valve already starts opening before the exhaust valve is completely closed. During that overlap period, the intake manifold’s vacuum can suck exhaust gas backward into the intake manifold, especially while idling.
With race car cams, this effect can be so extreme that the engine idles like a two-stoke engine.
This is why carbon and other combustion chamber deposits find their way into the intake ports. Are these deposits harmful? Probably not.