On a similar note, I've thought about putting together a presentation on debugging and understanding large codebases by fixing a bug on a large project whose source code I've literally never seen before. I figure that by picking a codebase I have 0 experience with, there's less risk of skipping over steps.
The biggest hurdle for me has been actually finding a project I'm interested in where I haven't yet gone poking around the source code for some reason or another.