Which of the following is a common mistake in fading hair?

Creating distinct lines instead of smooth transitions is critical to achieving a well-executed fade in hairstyling. A fade should have a seamless blend where the hair gradually transitions from short to long lengths, without harsh lines or stark contrasts. When distinct lines are present, it indicates a lack of blending techniques, making the fade appear unprofessional and uneven.

In contrast to this, the use of too much product can often be controlled or adjusted during styling, and cutting too many layers may affect the overall style but does not specifically pertain to the fading technique. Neglecting the hairline might be a concern for the overall finished look, but it does not specifically relate to the blending process in fading. Therefore, smooth transitions are essential, and avoiding distinct lines is key in mastering the fade technique.