what you say, sounds like a guy with a 2 ft long pony tail,could get tested(hair)and have the end of his ponytail pass...and the begining of it fail.
THC stops depositing in the hair 3-4 days after the donor begins abstination from smoking. Therefore, the hair closest to the scalp starts becoming "clean" while the ends of the hair remain ''dirty'' until its cut off. So when facing a hair test, it is adviseable to keep a regular hair cutting schedule and not keep a 2 ft (or longer) pony tail.