38 CFR § 20.1403 - Rule 1403. What constitutes clear and unmistakable error; what does not.
(a) General. Clear and unmistakable error is a very specific and rare kind of error. It is the kind of error, of fact or of law, that when called to the attention of later reviewers compels the conclusion, to which reasonable minds could not differ, that the result would have been manifestly different but for the error. Generally, either the correct facts, as they were known at the time, were not before the Board, or the statutory and regulatory provisions extant at the time were incorrectly applied.
(b) Record to be reviewed -
(1) General. Review for clear and unmistakable error in a prior Board decision must be based on the record and the law that existed when that decision was made.
(2) Special rule for Board decisions on legacy appeals issued on or after July 21, 1992. For a Board decision on a legacy appeal as defined in § 19.2 of this chapter issued on or after July 21, 1992, the record that existed when that decision was made includes relevant documents possessed by the Department of Veterans Affairs not later than 90 days before such record was transferred to the Board for review in reaching that decision, provided that the documents could reasonably be expected to be part of the record.
(c) Errors that constitute clear and unmistakable error. To warrant revision of a Board decision on the grounds of clear and unmistakable error, there must have been an error in the Board's adjudication of the appeal which, had it not been made, would have manifestly changed the outcome when it was made. If it is not absolutely clear that a different result would have ensued, the error complained of cannot be clear and unmistakable.
(d) Examples of situations that are not clear and unmistakable error -
(1) Changed diagnosis. A new medical diagnosis that “corrects” an earlier diagnosis considered in a Board decision.
(2) Duty to assist. The Secretary's failure to fulfill the duty to assist.
(3) Evaluation of evidence. A disagreement as to how the facts were weighed or evaluated.
(e) Change in interpretation. Clear and unmistakable error does not include the otherwise correct application of a statute or regulation where, subsequent to the Board decision challenged, there has been a change in the interpretation of the statute or regulation.