The Breakfast Baconator includes a grilled sausage patty, two slices of American cheese, six slices of applewood-smoked bacon, a fresh-cracked egg, and Swiss cheese sauce on a toasted brioche bun. It and the Maple Bacon Chicken Croissant are the most expensive breakfast sandwiches that Wendy's currently offers (they're both $4.39 in my area).
They also have another limited-time deal through the app where you can get a Double Stack for $1 with any purchase.
Photo via Wendy's.