The contextual interference (CI) effect refers to the learning benefits that occur from a random compared with blocked practice order. In this article, the cognitive effort explanation for the CI effect was examined by investigating the role of error processing. In 2 experiments, a perceptual-cognitive task was used in which participants anticipated 3 different tennis skills across a pretest, 3 practice sessions, and retention test. During practice, the skills were presented in either a random or blocked practice order. In Experiment 1, cognitive effort was examined using a probe reaction time (RT) task. In Experiment 2, cognitive effort was manipulated for 2 groups by inserting a cognitively demanding secondary task into the intertrial interval. The CI effect was found in both experiments as the random groups displayed superior learning in the retention test compared with the blocked groups. Cognitive effort during practice was greater in random compared to blocked practice groups in Experiment 1. In Experiment 2, greater decrements in secondary task performance following an error were reported for the random group when compared with the blocked group. The suggestion is that not only the frequent switching of tasks in randomized orders causes increased cognitive effort and the CI effect, but it is also error processing in combination with task switching. Findings extend the cognitive effort explanation for the CI effect and propose an alternative hypothesis highlighting the role of error processing.