[richardw]

I've had about four situations in my current project (Java/GWT) where going through the source has helped me understand what the developers expect, or have TODO comments for unimplemented bits.

In a previous project (jQuery/Python) I had an issue with a Python library that I needed to patch. It worked fine on my collaborator's Mac but not on my machine.

Even .net - I created a downloadable product that happened to require a fair amount of custom UI work and being able to look under the hood (with Reflector) helped a lot. There are still a couple issues that I could never get resolved that are buried in the stack.