This Bible contradiction is from the Skeptic's Annotated Bible.
Only Levites before the tabernacle.
Leviticus 17:1-5 View context
And the LORD spake unto Moses, saying,
Speak unto Aaron, and unto his sons, and unto all the children of Israel, and say unto them; This [is] the thing which the LORD hath commanded, saying,
What man soever [there be] of the house of Israel, that killeth an ox, or lamb, or goat, in the camp, or that killeth [it] out of the camp,
And bringeth it not unto the door of the tabernacle of the congregation, to offer an offering unto the LORD before the tabernacle of the LORD; blood shall be imputed unto that man; he hath shed blood; and that man shall be cut off from among his people:
To the end that the children of Israel may bring their sacrifices, which they offer in the open field, even that they may bring them unto the LORD, unto the door of the tabernacle of the congregation, unto the priest, and offer them [for] peace offerings unto the LORD.
Numbers 18:6-8 View context
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.
Therefore thou and thy sons with thee shall keep your priest's office for every thing of the altar, and within the vail; and ye shall serve: I have given your priest's office [unto you] as a service of gift: and the stranger that cometh nigh shall be put to death.
And the LORD spake unto Aaron, Behold, I also have given thee the charge of mine heave offerings of all the hallowed things of the children of Israel; unto thee have I given them by reason of the anointing, and to thy sons, by an ordinance for ever.
Non-Levites may offer sacrifices away from the tabernacle.
1 Samuel 1:1-2 View context
Now there was a certain man of Ramathaim-zophim, of mount Ephraim, and his name [was] Elkanah, the son of Jeroham, the son of Elihu, the son of Tohu, the son of Zuph, an Ephrathite:
And he had two wives; the name of the one [was] Hannah, and the name of the other Peninnah: and Peninnah had children, but Hannah had no children.
1 Samuel 7:7-9 View context
And when the Philistines heard that the children of Israel were gathered together to Mizpeh, the lords of the Philistines went up against Israel. And when the children of Israel heard [it], they were afraid of the Philistines.
And the children of Israel said to Samuel, Cease not to cry unto the LORD our God for us, that he will save us out of the hand of the Philistines.
And Samuel took a sucking lamb, and offered [it for] a burnt offering wholly unto the LORD: and Samuel cried unto the LORD for Israel; and the LORD heard him.