This study investigates how cognitive load and task complexity affect users' responses to security alerts. Drawing on Cognitive Load Theory and the Theory of Planned Behavior, we examine both unintentional errors – slips & lapses when users encounter security warnings. Seventy University students participated in a lab-controlled in-basket experiment, completing 40 tasks and responding to randomized security alerts. Behavioral responses, response times, and self-reported measures were collected. Results show that higher cognitive load leads to more ignored and negative responses, while lower cognitive load is associated with quicker and more accurate responses. Future work will incorporate gaze direction and micro expression analysis to further classify user decision-making behaviors.