Most difficult proof reading

What is the one book almost everyone goes through in their lifetime but is the hardest to proof read?

The Dictionary!

Unlike technical books, where even a small group of experts can have an authority on the matter, the dictionary contains thousands of areas of words and no small group of experts can proof read and verify what is written.

I wonder how the dictionary companies like Oxford dictionary, Webster's disctionary, etc actually proof read them. Moreover, in today's internet world, do they just see the new words used in Google/Yahoo and add them to the next edition? How do they find new words that are being used? Do they even rely on unconfirmed research from unreliable sources to and add the new words based on these researches?

I have my questions and reservations. Further, I wonder what will the daily work of a person working in these dictionary companies. What kind of resources do they have at their disposal? Do they have huge libraries & online databases? Do they go to different countries to see the frequency of usage of certain words? What will one have to do/be to get into these companies to work on the core of their work? I wonder...