Contradiction with Numbers 18:6
Numbers 4:49 restricts service to Levites aged 30-50, while Numbers 18:6 allows for a broader service of Levites without such age restrictions.
Numbers 18:6: And I, behold, I have taken your brethren the Levites from among the children of Israel: to you [they are] given [as] a gift for the LORD, to do the service of the tabernacle of the congregation.
Contradiction with 1 Samuel 8:11
Numbers 4:49 implies a religious service role for Levites, whereas 1 Samuel 8:11 describes taking sons for military and secular service.
1 Samuel 8:11: And he said, This will be the manner of the king that shall reign over you: He will take your sons, and appoint [them] for himself, for his chariots, and [to be] his horsemen; and [some] shall run before his chariots.
Contradiction with 2 Chronicles 31:17
Numbers 4:49 strictly enumerates only Levites serving, but 2 Chronicles 31:17 includes priests in service roles.
2 Chronicles 31:17: Both to the genealogy of the priests by the house of their fathers, and the Levites from twenty years old and upward, in their charges by their courses;
Contradiction with Ezra 3:8
Numbers 4:49 notes a specific age range for service, while Ezra 3:8 describes Levites beginning service at age 20.
Ezra 3:8: Now in the second year of their coming unto the house of God at Jerusalem, in the second month, began Zerubbabel the son of Shealtiel, and Jeshua the son of Jozadak, and the remnant of their brethren the priests and the Levites, and all they that were come out of the captivity unto Jerusalem; and appointed the Levites, from twenty years old and upward, to set forward the work of the house of the LORD.