In the episode when Homer sees the alien check out the part where the guy is selling the t-shirts. He tells Homer that the "Homer is a Dope" t-shirts are sold out, but then Homer sees Marge and Maggie wearing the t-shirt and Marge tells him about how great the shirts are. Homer turns to the vendor and asks for two t-shirts when the guy just told him that they were sold out.

If this is a joke (because Homer is supposed to be dumb), then it's not a very obvious one and it's not all that funny, either.