The most common complication of untreated radial head dislocation is cubitus valgus. This condition occurs when the elbow joint becomes abnormally angled, leading to a misalignment of the arm and an increased risk of further injury. Other complications such as damage to the ulnar nerve or myositis ossificans can also occur, but cubitus valgus is the most typical outcome in cases of untreated radial head dislocation.