The Mishnah (91b-92a) on our daf (page) discusses a number of cases where someone cannot participate in Temple activities because of his involvement in mourning practices. In a case where this limitation is only of Rabbinic nature, the Sages did not insist that their enactment be kept in the face of the mitzvah of korban Pesah.
There are two cases discussed in the Mishnah:
1. An onen - someone who lost a close relative that day
2. A melaket atzamot - someone who was involved in gathering the bones of a relative for final burial.
With regard to the onen, the halakha is that he has a unique status that forbids his participation in eating korbanot the first day on which a close relative died. This law is learned from the story of Aharon ha-kohen whose oldest sons, Nadav and Avihu were killed on the first day of the consecration of the mishkan, the Tabernacle (see Vayikra 10:19). By that night the Torah would permit him to eat korbanot, but the Rabbis forbade him from doing so. This rule applies to all sacrifices, except for the korban Pesah, since skipping the korban Pesah is not merely missing out on the fulfillment of a positive commandment, but potentially involves a serious punishment – karet (excision).
A melaket atzamot is someone who collects the bones of his relatives for final burial. During the Second Temple period - and for hundreds of years after that - there was a unique tradition with regard to burial. People were buried in the ground in plots that were designated as temporary resting places. After a number of years, when the flesh had decomposed and only the bones remained, they would be gathered and placed in an ossuary, a stone box, which would be interred in the family burial cave.
Although the gathering of the bones took place well after the death of the deceased, the day on which it took place was considered a day of mourning. The Gemara points out that we must be talking about a case where someone else did the actual gathering, since the person who did so would not be able to participate in the korban Pesah for a different reason - because he is tameh (ritually defiled).