Formative assessment in science has long been reported to support students' scientific understanding. It involves teachers gathering assessment evidence, interpreting it in the light of learning goals and providing feedback to bridge the gap between students' current state of understanding and the desired educational goals. This study examines how problems in planning and implementation of an assessment task, in the context of a unit on asexual reproduction in secondary school science, may result in loss of its learning potential. It is suggested that beneath incorrect responses lie distinct cognitive rules used by students. It is crucial to pay attention to these rules and use this information to develop corrective instruction. A repertoire of skills to support teachers' interpretation of assessment evidence is identified.