Suppose you are trying to figure out whether evidence will be admissible at an upcoming federal trial. Obviously, the best place to start is with the text of the Federal Rules of Evidence. But if you think that the text of a particular rule plainly answers the question of admissibility, or simply leaves the question open for argument, you will often be wrong. The case law has diverged from the text of the Federal Rules on many important points.
This divergence comes in two forms: (1) where the case law–defined as at least a fair number of reported cases–is flatly inconsistent with either the text of the Rule, the Advisory Committee Note explaining the text, or both; and (2) where the case law has provided significant development on a point that is not addressed by either the text of the Rule or the Committee Note.