We examine the learning curves of individual software developers in Open-Source Software (OSS) Development. We collected the dataset of multi-year code change histories from the repositories for five open source software projects involving more than 100 developers. We build and estimate regression models to assess individual developers' learning progress (in reducing the likelihood they may make a bug). Our estimation results show that developer's coding experience does not decrease bug ratios while cumulative bug-fixing experience leads to learning progress. The results may have implications and provoke future research on project management about allocating resources on tasks that add new code versus tasks that debug and fix existing code. We also find that different developers indeed make different kinds of bug patterns, supporting personalized bug prediction in OSS network. We found the moderating effects of bug types on learning progress. Developers exhibit learning effects for some simple bug types (e.g., wrong literals) or bug types with many instances (e.g., wrong if conditionals).