Old story from Ner Yisroel. 2nd seder, the Mashgiach looks around, frowning, then leaves the Beis Midrash. A bochur runs to the pay phones to call the dorms and warn them, saying, "The Mashgiach is going to the dorms!" After a pause, the Mashgiach (who had picked up the phone at the dorms) says, "Er is shoin du."