i imagine how in the year 3000 the people will be arguing why the people of the 21st century, i.e. we, wouldn't build say a fusion engines and power generators (we do have "novelty" fusion devices) , or may be even FTL space ship when it happens to be such a simple thing :) We probably have the tech for the fusion, or very close to it, yet we don't know how to put it together and/or in what direction to make that small final push.