When attention is otherwise engaged, observers may experience inattentional blindness, failing to notice objects or events that are presented in plain sight. In an inattentional blindness experiment, an unexpectedstimulus ispresented alongside primary-task stimuli, and its detection is probed. We evaluate a criterion that is commonly used to exclude observers from the data analysis. On the final experimental trial, observers do not perform the primary task, but instead look for anything new. Observers who fail to report the unexpected stimulus on thisfull-attention trialare excluded. On the basis of 4 hypothetical experiments and a review of 128 actual experiments from the literature, we demonstrate some potentially problematic consequences of implementing the full-attention-trial exclusion criterion. Excluded observers may cluster in experimental conditions and the exclusion criterion may lead researchers to understate the pervasiveness of inattentional blindness. It may even render usblindto inattentional blindness on the full-attention trial.