I once spent three days trying to figure out an issue, stepping line by line through hadoop (after figuring out the issue was in hadoop and not my own code). Yay, I proved the issue was actually in Java itself. Guess what happened next? We avoided the bug. Why?

- We couldn't update Java.

- We couldn't change hadoop because we were using a packaged solution. So, we just filed a bug with them.

Had the source not been available, we would have just skipped all of that, and it would have been our vendor's problem 3 days earlier.