Posters by Hero Design Studio. Watch a slideshow featuring some of the highlights from Bumbershoot’s Flatstock 18.One of the most enjoyable exhibits every year at Bumbershoot is Flatstock, featuring poster designs from artists around the country. One of the artists, Tad Carpenter, has a show, “Mayhem, Mischief and Monsters,” opening at Bluebottle Art Gallery (415 East Pine), on Tuesday. It runs through Sept. 29.