Designing and interpreting controlled experiments are important inquiry skills addressed in many current science curricula. The relevant skills associated with the design and interpretation of controlled experiments are summarised under the term control-of-variables strategy (CVS). Research on elementary school students' CVS skills shows that they have basic conceptions of CVS, as they can correctly identify controlled experiments to test given hypotheses and can interpret the results of controlled experiments. However, they perform poorly when they have to plan experiments and interpret confounded experiments. Furthermore, previous research has identified eight misleading preconceptions regarding CVS that cause invalid experimental designs, called design errors. The current study investigates the occurrence and change of design errors during elementary school and their consistency over different tasks. Via a latent class analysis (LCA), we identified three distinct patterns of design errors: (1) correct CVS understanding, (2) change of too many variables and (3) non-contrastive experiments. Our results show that many students as low as grade two have a basic understanding of CVS and that occurring design errors are associated with limited metaconceptual knowledge about when and why to apply CVS. We discuss possible implications of our findings for teaching CVS and further research.