The Rambam (korban pesach ch. 5) writes that a peson who missed Pesach Rishon b'ones and then b'meizid skips Pesach Sheni is chayav kareis. Yet, a person who was b'derech rechoka, too far from Yerushalayim to offer Pesach Rishon, and then skips Pesach Sheni b'meizid, is not chayav kareis. Ra'avad asks: why should there be any difference between these two cases?
Lomdus fans will like this one. Apparently the Rambam held that the ptur of ones and the ptur of derech rechoka are categorically different. Someone who was ones had a chiyuv to bring Pesach Rishon but was exempt due to circumstance; failing to make-up the chiyuv on Pesach Sheni renders one liable kareis for missing that original chiyuv. Someone who was b'derech rechoka is considered to have had no obligation whatsoever to bring Pesach Rishon. The opportunity to offer Pesach Sheni is a new obligation which does not carry the penalty of kareis.