Most software systems are complex and composed of a large number of artifacts. To realize each different artifact specific techniques are used resorting to different abstractions, languages and tools. Successful composition of different elements requires coherence among them. Unfortunately constraints between artifacts written in different languages are usually not formally expressed nor checked by supporting tools; as a consequence they can be a source of problems. In this paper we explore the role of the relations between artifacts written in different languages by means of a case study on the Hadoop open source project. We present the problem introducing its terminology, we quantify the phenomenon and investigate its relation with defect proneness.