I want to second what div already said, with the added comment that it's better to have a specific goal than to wander aimlessly through the source.

Every piece of software has bugs and limitations. When you encounter one, go figure out why it is the way it is, and maybe even fix it.

If all your dependencies are so perfect that this never comes up (hah!), you can always pick a project you use and like and look at their open issues. Begin trying to solve one of them.