Contradiction with Numbers 3:12
Numbers 3:12 states, "And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine." This suggests that God took the Levites instead of the firstborn, while Numbers 8:18 emphasizes that the Levites themselves are a substitution.
Numbers 3:12: And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;
Contradiction with Exodus 13:2
Exodus 13:2 commands, "Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine." This implies that God originally claimed the firstborn, contradicting the Levites being taken in Numbers 8:18 as a replacement.
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.