Why is Kalev seeking someone to conquer Kiryas Sefer? He was given only Chevron!
Maharsha (Temurah 16a): This is why Chachamim explained this as parables. Also, it is because it is repeated in Shoftim, Perek 1 (also Malbim Shoftim 1:10 says so).
How could Kalev promise his daughter to the one who will conquer the city? Perhaps it will be someone forbidden to marry into Yisrael, e.g. a Mamzer!
Vayikra Rabah 37:4: Indeed, Kalev erred. Even so, Hashem sent him Osni'el, a most worthy man to conquer the city and marry Ochsah.
R. Chaim Paltiel (Bereishis 24:14): Ta'anis 4a does not list Kalev among those who asked improperly, for "conquering the city alludes to recovering the Halachos that were forgotten in the mourning over Moshe. Surely a Mamzer would not merit this! 1
A Mamzer can be a Chacham (Horiyos 13a), but he would not merit such an awesome level of Chachmah! (PF)