Oil in coolant reservoir.
In this #Shorts video I show how after a major oil cooler failure there is major amounts of oil in the cooling system. This one was so bad it was pressurizing the coolant expansion tank and pump oil and coolant out of the overflow in the tank. If you have an issue like this it will repairs including major flushing afterwards to clean all the oil out.
Be sure to comment if you would like to see more like this :)