Model debugging is an important component of assisting modelers with constraint-based problem formulation. This paper is built around a case study in modeling a special class of CSPs, which represent problems that change when certain conditions are met. The control of changing the problem, by activating or deactivating variables, is part of the problem representation and is modeled through special constraints, called activity constraints. The activity constraints may interact with the other constraints and generate inconsistencies or redundancies. We present initial examples of these two types of iteractions, and we derive more general forms of inconsistency and redundancy. We believe this work can lead to methods for automatic model debugging, which detect and resolve problems with existing models.