During a trial, jurors may be asked to compare the defendant with video of the culprit. Yet matching unfamiliar people in this manner can be difficult and error‐prone. Judges may warn jurors about the dangers of identification from video images, but the effect of warnings is unknown. Here we report two studies that assessed the effect of warning instructions on defendant‐culprit matching. In Experiment 1, college students read either control, warning, or illustrated warning instructions before deciding if a defendant seen in one video matched the culprit in another video. Experiment 2 compared control and warning instructions in a sample of Amazon Mechanical Turk workers. Matching accuracy in both experiments was poor, and warnings did not consistently improve performance or make participants more cautious. These results do not support the supposition that warning instructions help jurors compare the defendant to images of the culprit.Copyright © 2016 John Wiley & Sons, Ltd.