The Rambam writes:
One who willfully nullifies this mitzvah and allows the fourteenth of Nisan to pass without him offering the Korban Pesach despite the fact that he is not impure or not on a distant journey, he is liable for kares. If he neglected the offering of this Korban inadvertently, he is exempt.
What is the chiddush that one is פטור if he is שוגג?? We would know that without the Rambam!!