Bechorot 5: Count of the Levi'im

The Gemara tells us that a Roman, Kontrokos, asked Rabban Yochanan ben Zakkai, "in the detailed record of the numbering of the Levites, the total is twenty-two thousand three hundred, but the sum total is only twenty-two thousand.  Where are the other three hundred?"  Rabban Yochanan responded that the sum total only includes the Levites who redeemed the Israelite firstborn.  The remaining three hundred were Levite firstborn - and one such person cannot relinquish the sanctity of his status. Abaye comments it is enough for a Levite firstborn to abrogate his own firstborn sanctity. 

Counting by family Gershom initially counted seven thousand and five hundred (Bamidbar 3:22).  The families of Kehat were eight thousand and six hundred (Bamidbar 3:28) and those of Merari were six thousand and two hundred (Bamidbar 3:28).  The total was twenty-two thousand and three hundred.  At the end of the counting, three hundred were missing.  Rabban Yochanan ben Zakkai explains that the last number is an introduction to the following discussion in the Torah.  That discussion is about the the requirement of exchanging Levi'im for the firstborn bechorim.  In the Talmud Yerushalmi, this discussion as about the participation of the firstborn in sacrificing the Golden calf, and the Levites did not participate in that transgression. 

The Maharsha notes that this was not an innocent question of Kontrokos.  It was an accusation that Moshe Rabbeinu had purposefully reduced the number of Levites so that more firstborn would need to pay the redemption fee of five shekels.  Moshe's brother Aaron would have been the beneficiaries of this deceptive move.  It is notable that the stereotype of Jew as money focused.