Mishnah Pesachim 10:3 says:
מְטַבֵּל בַּחֲזֶרֶת, עַד שֶׁמַּגִּיעַ לְפַרְפֶּרֶת הַפַּת
[he] dips the lettuce until he reaches parperet hapat
What exactly does parperet hapat mean in this context? Various commentators seem to agree that it's a part of the seder, but they disagree on which part.