The little previous research comparing student errors across schools indicates that student “bugs” do not transfer – that is, the distribution of students’ systematic errors in one school does not significantly match those in other schools. The issue has practical implications as cognitive (or “model-tracing”) tutors rely on the modeling of student errors in order to provide targeted remediation. In this study we examine the responses of students at three schools to a middle-school mathematics problem. We find the same error is the most common error across all schools, and this single error accounts for some half of all incorrect responses at each school. The top five errors are similar across schools and account for some 2/3 of errors at each school. We conclude that in this example, there appears to be considerable overlap of student errors across schools.