Gojko Adzic – The waterfall trap for “agile” projects:

"Because increments are done in detail, a lot of effort is wasted when a piece needs rework (and the initial releases are almost certain to fall into this category). Iterative development offers a chance to see the picture from the start, and guide the development towards the full picture in steps. Not carving stuff in stone from the start allows us to change them easier later on, and we know that we’ll need to do that. Jeff gave the following rule of thumb to check quickly if your plan is iterative or incremental: “it’s not iterating if you do it only once”."