Habits are automatic behaviors triggered by specific cues and are thought to optimize daily activities by reducing cognitive effort and enabling efficient and fast performance. Yet, they can also lead to inflexibility, preventing individuals from adapting to environmental changes. Since it has been difficult to examine habit formation in humans with traditional outcome devaluation paradigms, we applied a motor sequence learning task (MSLT) to study this process. Thirty-one participants (16 female, 28.4 ± 5.3 years old) completed the MSLT on two consecutive days. They implicitly learned to execute a 12-item motor sequence using four fingers, each corresponding to one of four distinct visual stimulus locations. Test blocks introduced sequence deviations by intermittently omitting one item of the sequence. We measured whether participants were able to flexibly adapt their behavior or would incorrectly execute the omitted response – a so-called action slip. Action slips serve as an indicator of automatization or behavioral inflexibility. Findings indicate that prolonged training led to faster response times and lower error rates in learning compared to random blocks, suggesting successful sequence learning and the emergence of automatic behaviors. Action slips increased with extensive training, demonstrating the shift towards automatic and inflexible responding, indicative of habit formation. The results highlight the utility of the MSLT in studying habit formation in humans and emphasize the role of extensive training, motor skills, and automaticity. The task offers a promising framework for investigating the neural and cognitive mechanisms underlying habitual behavior, providing new insights into the balance between habitual and goal-directed control.