Frequent Swift collaborator Aaron Dessner has surprised Swift’s crowd at three Eras Tour concerts so far. At the April 14 show in Tampa, he joined Swift to perform “The Great War” during the surprise portion of the show. The song is a “3am” track from Swift’s 2022 release “Midnights.” The next day, he returned to perform “Mad Woman” with Swift. That song is from 2020’s “Folklore.” He returned again on May 7 in Nashville to perform “Would’ve, Could’ve, Should’ve,” another…