Contradiction with Exodus 13:2
While Exodus 13:2 states that all firstborns belong to God and should be consecrated to Him, Numbers 18:16 discusses a method for redeeming the firstborn through monetary value, which may be seen as a contradiction in terms of direct consecration versus redemption.
Exodus 13:2: Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, [both] of man and of beast: it [is] mine.
Contradiction with Leviticus 27:26
Leviticus 27:26 says that no man can dedicate the firstborn of the animals as it belongs to the Lord already, which contrasts with Numbers 18:16 where a monetary redemption is involved for the firstborn of humans.
Leviticus 27:26: Only the firstling of the beasts, which should be the LORD'S firstling, no man shall sanctify it; whether [it be] ox, or sheep: it [is] the LORD'S. [firstling of: Heb. firstborn, etc]