The accelerating integration of artificial intelligence into materials selection processes has brought unprecedented efficiency to high-throughput screening and discovery campaigns, yet it has also introduced a subtle but profound failure mode that remains largely unrecognized in the field: scientific regret. This paper identifies scientific regret as a distinct failure mode in AI-driven materials science—the ex-post realization that a better material or research direction was passed over due to an AI recommendation, often under conditions of irreducible uncertainty and vast combinatorial search spaces. Unlike traditional statistical errors, scientific regret captures the experiential and consequential dimension of missed opportunities in research trajectories that are difficult or impossible to reverse. Drawing on foundational work in decision theory and recent advances in Bayesian optimization for materials discovery, the paper defines scientific regret, delineates its mechanisms of production within AI systems, develops a typology tailored to materials contexts, and outlines principles for its detection and mitigation. By analyzing how premature search space pruning, overconfidence in negative predictions, and misaligned acquisition functions contribute to regret, this analysis reveals how current AI paradigms may systematically undervalue exploration in favor of short-term gains. The implications for materials AI practice are significant, calling for the design of regret-sensitive systems that better balance exploitation with the long-term costs of locked-in choices. Ultimately, embracing scientific regret as a core design constraint promises to foster more robust, reflective, and innovative approaches to autonomous materials research. Scientific regret is not merely an abstract philosophical concern but a practical barrier to genuine progress in materials science. When AI systems guide researchers away from promising chemistries or structures, the subsequent realization of a missed opportunity can stall entire research programs, waste limited experimental resources, and distort the collective knowledge base of the field. This failure mode is especially insidious because materials discovery operates in enormous design spaces where exhaustive enumeration is impossible and where negative predictions are rarely revisited once resources are committed elsewhere. By foregrounding scientific regret as a failure mode, this analysis seeks to reorient the community toward decision frameworks that explicitly account for the irreversible nature of many AI-influenced choices in materials selection.