Post-Implementation - Most Important
http://infocentre.frontend.com/infocentre/articles/postimplementationmostimportant.html

Farrell, Tom
Frontend Infocentre
2000

Abstract:

You would be forgiven for thinking that the lifespan of the typical usability project ends with final implementation and a product or website's release into the market. In one sense it does - the user-centred design process is over, and presumably the result is an easy-to-use product or site that is already yielding results. All well and good - but sometimes even the best interfaces cannot meet the changing requirements of the typical workplace. In this case, post-implementation user testing can pick up the difficulties that were not picked up first time around. At this point the client may quite rightly be wondering why these problems weren't spotted before deployment. The reason lies in the way in which the user has developed alongside the product. First impressions, whether favourable or not, may turn out to be misleading after an extended period of use.