The fossil record of drilling predation has been widely used to study predator-prey interactions and their relative importance on long-term evolutionary processes. Incomplete drill holes have been interpreted as indicators of failed attacks due to well-defended prey. However, this interpretation is based on pair-wise interactions between one predator and one prey, a condition commonly compromised in nature. The hypothesis that interference among drilling predators leads to an increase in the relative frequency of incomplete drill holes was tested in the laboratory using the naticid Neverita delessertiana (R cluz) and a common prey, the bivalve Chione elevata (Say). The experiment consisted of an isolation treatment, where predators fed alone, and a competition treatment, where predators fed in groups of three. Predators in competition were grouped into two size cohorts, small and large. All drilling attacks made by isolated predators of both size groups were successful, resulting in complete drill holes, whereas, in competition, the incomplete drilling frequencies were 6.9% for the small predator group and 21.3% for the large predator group. A range of competitive, predator-predator interactions were observed, including grappling, prey theft, and cannibalism. These results suggest that interpretations of both field and fossil data must consider the role of competitive disruption as an additional source of incomplete drill holes. The implications of other observations, including prey 'suffocation' and the resumption of incomplete drill holes after successful prey theft, are also discussed.