Does this sight look familiar to you? This is what happens when your oil gets too hot and burns in the frying pan. Unfortunately, the bitterness does not end there. When oils are heated to or beyond their smoke point, they become oxidized. Oxidation creates free radicals (potential cancer causing molecules). Saturated fats have high smoke points, therefor they should be used when cooking hot food. Polyunsaturated, and monounsaturated fats are less stable and reach their smoke points at lower temperatures.
The moral of the story, don't use olive oil for everything. Use appropriate oils for appropriate cooking methods. Saturated fats like clarified butter, ghee, lard and coconut oil are great for high temperature cooking.
Oils may also experience increased oxidation from sitting out exposed on your counter top. To reduce rancidity, refrigerate your polyunsaturated, and monounsaturated fats or keep air & light out. This means putting your sealed olive oil in the refrigerator or cupboard.