When I was assigned to correct some error in one day's processing, I examined every detail item by item, since I wasn't familiar with the system. While the people familiar wth the system could leap from point to point, quickly narrowing in on the error, I had to be as picky as an accountant, verifying everything was correct. As a consequence, I discovered several errors that had been there for years, that no one was aware of.

Moral of the story: the big picture is good, but details are important, especially when they're wrong.