Increasingly, online discussion communities are used to support activities ranging from software development to political campaigns. An important feature of an online discussion community is its content boundaries, which are individual perceptions of what materials and discussions are part of the community and what are not, and how that community is related to others within a larger system. Yet in spite of its importance, many community infrastructures allow individual participants to reshape content boundaries by simultaneously associating their contributions with multiple online discussion communities. This reshaping behavior is a controversial aspect of the creation and management of many types of online discussion communities. On one hand, many communities explicitly discourage boundary reshaping behaviors in their frequently asked questions or terms-of-use document. On the other hand, community infrastructures continue to allow such reshaping behaviors. To explain this controversy, we theorize how the extent of boundary reshaping in an online discussion community has simultaneously positive and negative effects on its member dynamics and responsiveness. We test predictions about the conflicting effects of reshaping behaviors with 60 months of longitudinal data from 140 USENET newsgroups, focusing on cross-posting activities as a form of reshaping behavior. Empirical results are consistent with the proposed hypotheses that reshaping behaviors within a discussion community affect member dynamics and community responsiveness in both positive and negative ways. Taken together, the findings highlight the boundary-related design challenges faced by managers seeking to support ongoing activity within online discussion communities.