Novice programmers' understanding of conditional and loop constructs are often incomplete. They seem to understand a single conditional or single loop, but fail to understand the combination of them. We propose a method for finding misconceptions underlying this failure. We first developed a tracing quiz set to locate the exact points at which students will fail. Second, we identified some misconceptions from experiments on five courses. Third, to use and validate these misconceptions, we developed an interactive test system which showed the correct answers to the students and requested them to describe their explanations. The experiments showed that some misconceptions affected the overall performance of the students.