פירוש על קידושין 142:3

This source presents the tension between Torah learning and lineage. The Torah learning in Eretz Yisrael is good enough such that Ze’iri moved there from Babylonia. But the lineage is not good enough, even for him to marry R. Yohanan’s daughter. R. Yohanan points out to him that there remain in Babylonia both fit Jews (priests, Levites and Israelites) and there remain there unfit classes (mamzerim and netinim). So marrying only those of Babylonian descent will not help.
But R. Yohanan forgot that which R. Elazar taught. Ezra sorted out the genealogical lines before he left Babylonia. Therefore, Ze’iri did not fear marrying one from Eretz Yisrael. Sort of ironic that R. Yohanan forgets that which another Eretz Yisraeli sage teaches.