I once turned down an accepted offer because I realized I was not qualified to perform the task. They wanted me to analyze and estimate the time required for a rewrite on technology I was totally unfamiliar with. The greenfield development was extremely attractive to me, but I was able to temper it by coming to terms with the fact that the stress of achieving their goals with my time and skills would be really irresponsible. There are other opportunities. Don't sacrifice your sanity.