Source: Michael Fishbane's Biblical Interpretation in Ancient Israel (New York: Oxford, 1988) 193.
Numbers 15:22-23 says the following:
"22 But if you unintentionally fail to observe all these commandments that the LORD has spoken to Moses--
"23 everything that the LORD has commanded you by Moses, from the day the LORD gave commandment and thereafter, throughout your generations" (NRSV).
According to Fishbane, v 23 is a later addition, which is designed to clarify v 22. V 22 says "all these commandments," which one can easily construe as the commands of Numbers 15:3-21. To correct that misconception, a later editor adds v 23, making clear that the principle applies to all of God's commandments, not only the ones in Numbers 15.
This makes sense, in my opinion. On the other hand, couldn't a single author write "all these commandments," then correct or clarify himself? "All these commandments" occurs in Leviticus 26:14, and it doesn't immediately follow specific laws. Maybe the author wrote "all these commandments" meaning all of the Torah (in his understanding of Torah), then offered further clarification, since he realized he could be misunderstood.