In open source software development, the collaboration among developers is the key to improve software quality. In particular, to fix a bug related to various parts of a system, developers need collaboration because each developer usually has very limited knowledge about a large software system. This paper aims to clarify how narrow (or how wide) is each developer's knowledge area in the Eclipse project, and how often do developers need to collaborate with each other. As a result of analysis, we found that 50 % of committers take care of just one or two modules, which indicates the necessity of collaboration when a bug-fix affects multiple modules. In addition, we also found the significant relationship between committers' collaborations and the re-opened bugs. We conclude that a committer should be aware the risk of re-opened bugs caused by the collaboration.