The paper investigates the generation of harmonics in adaptive feedforward cancellation schemes. Specifically, it is shown that an algorithm designed to reject a certain number of frequency components may in fact be capable of reducing higher-order harmonics as well. This effect originates in the time-variation of the adaptive parameters. A consequence is that the adaptive system may perform better than the algorithm with the fixed, ideal parameters. Surprisingly, the response to higher-order harmonics can be calculated precisely, using a Laplace transform analysis. The origin of the numerical procedure is in the equivalence between the adaptive feedforward cancellation scheme and a control scheme based on the internal model principle. The implication of the results on design and implementation issues is discussed.