Leaving oVertone on for too long can have a few adverse effects. First, it can cause hair damage due to the intense heat and chemical reactions occurring in the process of coloring your hair. Too much heat or prolonged exposure to bleaching agents such as hydrogen peroxide or ammonia can weaken and dry out the strands, leading to breakage and split ends. Additionally, leaving oVertone on too long may lead to patchy results because certain areas of the hair are exposed longer than others, causing uneven color distribution. Finally, keeping oVertone on past its recommended time frame could cause scalp irritation due to contact with harsh chemicals and high temperatures; this is especially true if you already have sensitive skin. To avoid these negative outcomes it's important that you follow all instructions carefully when using any at-home dye product so that you get the desired result without damaging your locks!