Update: Since I went to bed there is now a heated discussion on FlyerTalk about whether this is a “tricked” fare and should not be posted publicly. I agree with the OP in that thread, that while a YQ may be missing, there is no “extra” step required to accomplish the trick. It is a mistake fare just like the trip to Spain I took last year on a Spanair codeshare operated by US Airways.
Since mistake fares are going to disappear quickly anyway and do not involve tricks that can be used on future itineraries, I believe that they not only can but should be shared openly to let as many people get in on the deal. However, I have decided that if any original posters share a fare on FlyerTalk or elsewhere and later contact me to remove my post, I will try to fulfill his or her request as quickly as possible.
News flash! A mistake fare from Seattle to Madrid has been reported on FlyerTalk by flyinghigh77 at $305 all-in. I wasn’t able to find it that low, but there is a fare for $420 using March 4-12 as the travel dates. Three of the flights are on Delta with an initial leg from Seattle to La Guardia on Frontier.
Personally, I wouldn’t take this option since I’ve decided I can only tolerate international coach with the extra legroom I get from United, nor would this contribute toward my quest to renew my United 1K status. But it may be useful to some of you. I would recommend you look into a connecting flight to Barcelona, which is a far more interesting city than Madrid.
This fare is not showing up on ITA in the searches I’ve done, so it appears to be a very selective mistake that you should jump on quickly. Head to www.cheapoair.com to book.