"I" means Shakyamuni Buddha...If you confuse the general with the specific, even in the slightest, you will fail to attain Buddhahood.

"This excellent good medicine I now leave here. You should take it and drink it and not fear that you will not be cured. In the final dharma age, it means Namu Myoho Renge Kyo, "I" means Sakyamuni..." (STN 3:2666) 

"Bodhisattva Superior Practices received the water of the wisdom of the Mystic Law from the Thus Come One Shakyamuni and causes it to flow into the wasteland of the people’s lives in the evil world of the latter age. This is the function of wisdom. Shakyamuni Buddha transferred this teaching to Bodhisattva Superior Practices, and now Nichiren propagates it in Japan. With regard to the transfer of teachings, it is divided into two categories: general and specific. If you confuse the general with the specific even in the slightest, you will never be able to attain Buddhahood and will wander in suffering through endless transmigrations of births and deaths."

The biggest offender is the Soka Gakkai which teaches that we endowed Shakyamuni Buddha with the seed of Buddhahood rather than the ither way around. This is another reason why they fail to attain Buddhahood.