Blocking bugs are a severe type of bugs that prevent other bugs from being !xed. As software becomes increasingly complex and large, blocking bugs occur in many large-scale software, especially in software ecosystems. Blocking bugs may have a high negative impact on software development and maintenance. Usually, blocking bugs preventing more bugs should be more concerned. In this paper, we focus on a special type of blocking bugs that block at least two bugs, which we call Critical Blocking Bugs (CBBs). We study CBBs from the following !ve aspects: the importance, the repair time, the scale of repair, the experience of developers who repair CBBs, and the circumstance why CBBs block multiple bugs. We build a dataset containing !ve open source projects and classify bugs into three types (i.e., critical blocking bugs, normal blocking bugs and other bugs, which block at least two, just one and zero bugs, respectively) to compare the di"erences between CBBs and other types of bugs. The experimental results show that CBBs are more important with longer repair time and larger repair scale, and CBBs are concentrated on parts of components of the project. These results highlight that CBBs are di"erent from other types of bugs in many aspects, and we should pay more attention to such bugs in the future software maintenance process.