Why doesn't my code work? Instructors for introductory programming courses frequently are asked that question. Often students understand the problem they are trying to solve well enough to specify a variety of input and output scenarios. However, they lack the ability to identify where the bug is occurring in their code. Mastering the use of a full-feature debugger can be difficult at this stage in their computer science education. But simply providing a hint as to where the problem lies may be sufficient to guide the student to add print statements or do a hand-trace focusing on a certain section of the code. Herein we present a software tool which, given a C++ program, some sample inputs, and respective expected outputs, uses graph mining to identify which lines in the program are most likely the source of a bug. The tool includes a visual display of the control flow graph for each test case, allowing the user to step through the statements executed. Experimental results from a group of CS1 students show that practice with this method: (1) makes students faster at finding bugs, (2) improves the way students test a program, and (3) improves program comments by students.