Answer:
In such a situation both lungs and tissues would receive blood with the same amounts of oxygen, which would affect the efficiency of the delivery of O2 to the tissues of the body. Generally, it occurs when there's a ventricular septal defect (a hole between the two ventricles), that causes the poorly oxygenated and well oxygenated blood to mix resulting into semi oxygenated blood.
It results in blood circulating in the arteries to be less oxygenated than it should due to which activity of the animal would be decreased.