False. While undercooking fish can certainly be a mistake, it is not necessarily the most common one. Overcooking fish is also a common mistake that can result in dry, tough, or rubbery flesh. Other mistakes when cooking fish might include using the wrong cooking method for the type of fish being prepared, not seasoning the fish properly, or not allowing the fish to come to room temperature before cooking. The most common mistake may vary depending on the individual cook and their level of experience.