This paper analyzes the effect of errors in antenna weights on the performance of adaptive array systems, both in the case when an array is used to maximize the gain in a desired direction and in the case when an array is used to null interfering signals. We begin by deriving an explicit characterization of the loss in array gain due to phase errors in the optimal antenna weights. Then, we examine interference rejection in the presence of amplitude and phase errors in the antenna weights. We prove that the loss in interference rejection is independent of the number of antennas. For both cases, we give numerical simulations that validate our analysis.