I will recommend that you go through Universities course pages on courses related to Operating Systems, Systems Programming, Computer Networks & Distributed Systems. Many of them have links to programming assignments and projects, try implementing those.
Advantages:
1. Problem will be well defined for you.
2. Better to implement eventual consistency between 3 nodes, distributed file system or single user database than trying to figure a bug in a large open source codebase.
You may find following links helpful in finding some of such courses: