During the COVID-19 sanitary crisis, many exams were hastily moved to online mode. This revived a much needed debate over the privacy issues associated with online proctoring of exams, while the validity and fairness of unproctored exams were increasingly questioned. With a randomized control trial, we estimate the effectiveness of prior warnings as a means of discouraging academic dishonesty in exams. We use original, non-intrusive technologies to surreptitiously identify cheating in a series of unproctored assignments and send a targeted warning to half of the students who were identified as cheaters. We then compare their cheating behavior on the final exam with the behavior of the group of unwarned cheaters. The warning proves effective but does not completely eliminate cheating, as some students’ cheating strategies become more sophisticated following issuance of the warnings. We conclude that switching traditional exams to online mode should be accompanied by proctoring. When proctoring is not possible, credible and effective anti-cheating technologies should be deployed together with adequate warnings.