Woordenboek der Nederlandsche Taal (Dictionary of the Dutch Language)
A dictionary of - you guessed it - the Dutch language, originally aiming at completeness.
Rumoured to be the largest dictionary in the world. Reputedly sound, but very far from complete; many perfectly ordinary words aren't in it.
By way of preparation, a spelling reform was designed and implemented. The first diictionary pages appeared in 1864; the first band (A-AJUIN) in 1882. By the time the first supplements appeared, it was clear that the amount of work on this dictionary just did not converge.
In 1976, it was decided to rush the project, by not collecting any material newer than 1921. Convergence ensued and the product was finished in 1998. At least two more spelling reforms have occurred since 1864; it is really funny to see a brand new book written in a spelling that hasn't been used in most people's lifetime.
Supplements and a completely new, electronic version are in the works. Details at http://www.inl.nl/WNT/.