It happened on August 23 of 2011. A team of mainly reserves (this isn't hyperbole) went down to Monterrey, Mexico and pulled off a 1-nil win. That Seattle Sounders side faced a fairly standard Rayados lineup. They won. It was an upset, but the stakes were not high. It damaged the mystique, but was not an advancement in a knockout stage. Seattle would wait until 2013 to make that history.
Let's review the lineup in light of the injuries to the current squad reducing our hopes. The number in brackets is the number of MLS starts (regular season and playoffs) the player had in 2011.
Terry Boss ; James Riley , Zach Scott , Jeff Parke , Tyson Wahl ; Roger Levesque  (Amadou Sanyang 84' ), Brad Evans , Servando Carrasco , Alvaro Fernandez  (Leo Gonzalez 76' ); Nate Jaqua  (Osvaldo Alonso 65' ), Pat Noonan 
Here is a collection of links from that match to remind you just what happened.