Contradiction with Exodus 28:1
1 Samuel 2:18 describes Samuel, a young boy, ministering before the LORD in a linen ephod, whereas Exodus 28:1 specifies that priesthood and the wearing of ephods were limited to Aaron and his sons.
Exodus 28:1: And take thou unto thee Aaron thy brother, and his sons with him, from among the children of Israel, that he may minister unto me in the priest's office, [even] Aaron, Nadab and Abihu, Eleazar and Ithamar, Aaron's sons.
Contradiction with Numbers 4:3
1 Samuel 2:18 refers to Samuel ministering as a child, while Numbers 4:3 sets the age requirement for Levites serving in the tabernacle from thirty years old and upward.
Numbers 4:3: From thirty years old and upward even until fifty years old, all that enter into the host, to do the work in the tabernacle of the congregation.
Contradiction with Numbers 3:10
Samuel ministered as a child before the LORD, yet Numbers 3:10 requires that non-priests who attempt to serve in the priestly role must be put to death.
Numbers 3:10: And thou shalt appoint Aaron and his sons, and they shall wait on their priest's office: and the stranger that cometh nigh shall be put to death.
Contradiction with Hebrews 7:12-14
1 Samuel 2:18 involves a young Samuel from the tribe of Ephraim ministering, whereas Hebrews 7:12-14 emphasizes the change in law because Jesus, from the tribe of Judah, could not be a priest under the old covenant.
Hebrews 7:12-14: For the priesthood being changed, there is made of necessity a change also of the law.