When Free Agent brings Twilight Sparkle home to introduce her to his parents, his mother refuses to believe that her son’s very special somepony is an alicorn princess, and must be a fake. She’s happy that he’s got a girlfriend, but treats Twilight as if she’s a changeling in disguise.
And the Iron E award goes to…
Which gives more point to the “That Twilight is actually another changeling” thing.