Pesachim, Chapter 9, Mishnah 1. This chapter deals with the various laws relating to Pesach Sheni - the second opportunity to bring the korban Pesach, when a person was unable to bring it on the fourteenth of Nisan.
The words of the Mishnah:
"Mi shehayah tamei o b'derech rechokah v'lo asah es harishon - ya'aseh es hasheni" - one who was tamei or on a distant journey and did not offer the first Pesach shall offer the second. These are the two situations the Torah lists as reasons why a person might not bring the korban Pesach at its proper time:
"Tamei" - a person who had contracted tumah from a corpse on the fourteenth of Nisan, and because of his tumah was unable to bring the korban Pesach.
"B'derech rechokah" - a person who was far from the Mikdash. We will yet address the question of exactly what distance is considered a "distant journey."
One who did not offer the first Pesach on the fourteenth of Nisan for one of these two reasons shall offer the second, and bring the korban Pesach on Pesach Sheni, the fourteenth of Iyar.
The Mishnah's addition - the unintentional and the compelled:
The Mishnah adds that not only one who was tamei and one who was on a distant journey are obligated in Pesach Sheni, but even a "shogeg o anus" - one who acted unintentionally or was compelled:
Shogeg - one who mistakenly did not realize that he was obligated to bring the korban Pesach.
Anus - one who was prevented by some other circumstance beyond his control, apart from tumah and a distant journey, from bringing the korban (for example, he was lying in a hospital).
They too, if they did not offer the first, shall offer the second.
This raises the question that the Mishnah itself asks: "Im kein, lamah ne'emar tamei o shehayah b'derech rechokah?" - if so, why does the verse specify one who was tamei or on a distant journey? Why did the Torah single out the one who was tamei and the one who was on a distant journey, if every shogeg and every anus has the opportunity to bring the korban Pesach on Pesach Sheni?
The Mishnah's answer:
The difference is not in the obligation of Pesach Sheni itself, but in the consequence of failing to bring it:
The one who was tamei and the one who was on a distant journey - are exempt from kareis. Even if in the end they do not bring the second Pesach, they do not bear responsibility and are not liable to kareis.
The shogeg and the anus - are liable to kareis. If they do not bring the second Pesach, and they deliberately failed to bring it, they become liable to kareis, since they are obligated in it.
The law of the deliberate offender:
One who acted entirely deliberately and does not bring Pesach Sheni is liable to kareis for having missed both of them, the first and the second alike.
Likewise, one who deliberately did not bring the first can still bring the second. But if he does not bring the second, even if he was prevented from doing so unintentionally, since the first time he acted deliberately he becomes liable to kareis for this, for he did not bring the korban at its proper time and did so deliberately.
In summary: In this Mishnah we learned that one who was tamei and one who was on a distant journey, and likewise the shogeg and the anus, who did not offer the first Pesach, shall offer Pesach Sheni on the fourteenth of Iyar. The reason the Torah specified tumah and a distant journey in particular is the difference in the law of kareis: these are exempt from kareis even if they do not bring the second, whereas the shogeg and the anus are liable to kareis if they do not bring it. And the deliberate offender, who brought neither the first nor the second, is liable to kareis for both.