Combinatorial test design (CTD) is an effective test planning technique that systematically exercises interactions between parameters of the test space. The test space is manually modeled by a set of parameters, their respective values, and restrictions on the value combinations. A subset of the test space is then automatically constructed so that it covers all valid value combinations of every$t$ parameters, where $t$ is a user input. This paper describes patterns that we have found to be recurring in combinatorial models, i.e., recurring properties of the modeled test spaces. These patterns are often hard to identify and capture correctly in a model, thus are common pitfalls in combinatorial modeling. We describe these patterns, supply methods for identifying them, and suggest simple yet effective solutions for them. © 2012 IEEE.