There's a famous dispute between R' Eliezer and R' Yehoshua when the world was created: Tishrei or Nissan. Their dispute is recorded in Rosh Hashanah 10b - 11a:
תניא רבי אליעזר אומר בתשרי נברא העולם בתשרי נולדו אבות בתשרי מתו אבות בפסח נולד יצחק בראש השנה נפקדה שרה רחל וחנה...
Rabbi Eliezer says: In Tishrei the world was created; in Tishrei the Patriarchs were born; in Tishrei the Patriarchs died; on Passover Isaac was born; on Rosh HaShana Sarah, Rachel, and Hannah were remembered by God and conceived...
ר' יהושע אומר בניסן נברא העולם בניסן נולדו אבות בניסן מתו אבות בפסח נולד יצחק בר"ה נפקדה שרה רחל וחנה...
Rabbi Yehoshua disagrees and says: In Nisan the world was created; in Nisan the Patriarchs were born; in Nisan the Patriarchs died; on Passover Isaac was born; on Rosh HaShana Sarah, Rachel, and Hannah were remembered by God and conceived sons...
As we can see from this baraisa, they both agree that Yitzchak was born on Pesach (since Sarah conceived on Rosh Hashanah).
Rav Chaim Kanievsky shlita, in his sefer LeMachseh Atik to Genesis 21:2, brings a dispute as to when Yitzchak was born. Seder Olam Chapter 5 and Midrash Tanchuma Vayeira § 17 say Pesach, yet the Zohar III parshas Beha’alosecha p. 151a1 says he was born on Rosh Hashanah.
His final comment is that this dispute is the same dispute found in Rosh Hashanah 10b between R' Eliezer and R' Yehoshua. Yet, as shown above, they both agree on this point! He even cites Rosh Hashanah 11a as a source that Yitzchak was born on Pesach (although he writes עי' ראש השנה, not sure his intention there...).
How can this be resolved?
Note: See the comments of the Turei Even, Poras Yosef (פור"י ד"ה בתשרי נולדו), and Aruch LaNer (ד"ה בתשרי מתו) to 11a, where it is clear they understood the gemarra to be saying everyone agrees that Yitzchak was born on Pesach.
1 Not sure if this helps but the sefer cites p. 302b, but on Sefaria there is no p. 302b. I found it on p.151a.