The end of Pesachim 9:8 says:
נִתְעָרֵב בִּבְכוֹרוֹת, רַבִּי שִׁמְעוֹן אוֹמֵר, אִם חֲבוּרַת כֹּהֲנִים, יֹאכֵלוּ:
If the Pesach got mixed up with Bechoros, Rabbi Shimon says, if it's a group of Kohanim they should eat it.
All of the procedures for bringing a Pesach and a Bechor are identical. However, a Bechor can be eaten for two days and a Pesach can only be eaten for one night. Rabbi Shimon allows bringing the unknown korbanos, even though the result is that the Bechoros can only be eaten for less time than ordinarily allowed. Although it's not explicit here, the Chachamim disagree with Rabbi Shimon (אין מביאין קדשים לבית הפסול), and the Rambam follows them.
בְּשַׂר חֲגִיגָה שֶׁעָלָה עִם הַפֶּסַח עַל הַשֻּׁלְחָן וְכָל הַתַּבְשִׁילִין הָעוֹלִים עִמּוֹ עַל הַשֻּׁלְחָן מִתְבַּעֲרִין עִמּוֹ. וְאֵינָן נֶאֱכָלִים אֶלָּא עַד חֲצוֹת כַּפֶּסַח עַצְמוֹ גְּזֵרָה מִפְּנֵי הַתַּעֲרֹבֶת:
Chagigah meat and other cooked foods that were on the table with the Pesach are burned with it. And they can only be eaten until midnight, like the Pesach. This is a decree because they might get mixed up.
Why are you allowed to put the Chagigah on the table with the Pesach when this limits the time it's allowed to be eaten?