One technique for identifying and addressing common student errors is the method of classroom voting, in which the instructor presents a multiple-choice question to the class, and after a few minutes for consideration and small group discussion, each student votes on the correct answer, often using a hand-held electronic clicker. If a large number of students have voted for one particular incorrect answer, the instructor can recognize and address the issue. In order to identify multiple-choice questions which are especially effective at provoking common errors, we recorded the percentages of students voting for each option on each question used in 11 sections of multivariable calculus, taught by four instructors, at two small liberal arts institutions, all drawing from the same collection of 317 classroom voting questions, over the course of 5 years, during which we recorded the results of 1,038 class votes. We found six questions in which, on average, more than 50% of each class voted for the same incorrect answer. Here we present these six questions and we discuss how we used them in the classroom in order to promote discussion and student learning. (Contains 7 tables and 7 figures.)