Source: Michael Fishbane's Biblical Interpretation in Ancient Israel (New York: Oxford, 1988) 191.
Fishbane believes that Numbers 15:25-26 expands on Leviticus 4:20. He thinks the italicized parts are the addition. (Citations are from the New Revised Standard Version.)
Leviticus 4:20: He shall do with the bull just as is done with the bull of sin offering; he shall do the same with this. The priest shall make atonement for them, and they shall be forgiven.
Numbers 15:25-26: The priest shall make atonement for all the congregation of the Israelites, and they shall be forgiven; it was unintentional, and they have brought their offering, an offering by fire to the LORD, and their sin offering before the LORD, for their error. All the congregation of the Israelites shall be forgiven, as well as the aliens residing among them, because the whole people was involved in the error.
But why should we assume that Numbers is interacting with Leviticus in this case? In Fishbane on Numbers 15, Part 1, I showed that Fishbane acknowledges clear differences in the two chapters on sacrificial details. Is Numbers interpreting Leviticus 4, or is it offering an explanation for what it understands as the sin offering? Moreover, maybe the line about atonement and forgiveness is a standard expression in the culture. I don't know. I guess the weakness there is that I can only imagine the line in a text, and why shouldn't we assume that text is Leviticus 4?