The Court stated that even for preserved errors, only those that structurally "undermine the fairness of a criminal proceeding as a whole" require reversal without a consideration of the effect the error had on the proceeding. The omission of a Rule 11 warning alone does not fall into this category. Furthermore, the Rule 52 phrase "error that affects substantial rights," which is used in Rule 52, "has previously been taken to mean error with a prejudicial effect on the outcome of a judicial proceeding." The Court concluded by holding that "a defendant who seeks reversal of his conviction after a guilty plea, on the ground that the district court committed plain error under Rule 11, must show a reasonable probability that, but for the error, he would not have entered the plea." The Court explained that a reasonable probability is one that is "sufficient to undermine confidence in the outcome" of the proceeding.