Herpes on a pap smear..
If you're in doubt, a smear can help. The herpes-infected epithelial cells are extremely large, and the nuclei are multiple, very swollen, and molded to each other. Compare the herpes cells to the normal-sized neutrophils which occur in the background. If you did this yourself, and recognized these cells, you'd say, "I've done a Tzanck test!"
Main lecture slide page