Final answer:
Overlapping lightener during a retouch can cause hair damage such as breakage and chemical burns because bleaching agents affect the structure of the hair and should only be applied to new growth.
Step-by-step explanation:
Overlapping lightener during a retouch service could cause significant damage to the hair. When lightener, which is a chemical bleaching agent, is applied to hair that has already been lightened, it can overlap onto previously treated hair. This can result in breakage, weakness, and even chemical burns on the scalp if not done carefully. It's important because bleaches react not only with pigments but also with the hair's natural structure, potentially leading to damage. To maintain hair health and integrity, it's critical to apply lightener only to new growth and avoid touch the already lightened hair.