Niddah 7 - Four Women with the Presumption of Purity
According to Rabbi Eliezer, there are types of women who, even if they discover blood, are not given ritual impurity retroactively. They are a virgin who never saw blood, a pregnant woman, a nursing woman, and an old woman. The Talmud goes on to define the exact age and time events for them.
Rabbi Yehoshua said, "I heard this law only in regard to a virgin, but not in regard the other three enumerated by Rabbi Eliezer." Rabbi Eliezer answered him, "You did not hear it from your teachers, but I did hear from mine. Do we ask to testify about a new moon the one who did not see it, or the one who saw it?" While Rabbi Eliezer was alive, the law followed Rabbi Yehoshua, but after Rabbi Eliezer's passing, Rabbi Yehoshua returned the law to follow Rabbi Eliezer.
Why did Rabbi Yehoshua do it? He reasoned that if they were to follow Rabbi Eliezer in this law, where his view was indeed the correct one, then people might follow Rabbi Eliezer even in other areas, where he might have been wrong. If that happened, Rabbi Yehoshua would not be able to stop them: Rabbi Eliezer was under a ban and forbidden to defend his view, and Rabbi Yehoshua would not feel right to speak when he was allowed to, while Rabbi Eliezer would be forbidden it, out of respect for Rabbi Eliezer. After Rabbi Eliezer's passing, this was no longer a concern.
Art: A Pieter De Hooch - Woman Nursing an Infant with a Child and a Dog