Some T-shirts are sold. Others are only available for a few weeks, in one room, on one street. An Aime Leon T-shirt from a pop-up falls in the second group. Aimé Leon Dore's temporary shops produced limited-run tees tied to a theme, a place, and a date, and that scarcity is what made them memorable. The takeaway: pop-up exclusives are the best expression of what the brand does well, but...