During the battle with the changelings, a trapped Discord manages an undirected spell that reaches across universes to bring an agent of chaos to Equestria. What's brought back more than qualifies.

Steam! Gears! SCIENCE! A very confused Spark! And, eventually, really good coffee.

Here comes Agatha Heterodyne--MARE GENIUS!

After the changelings' failed invasion of Canterlot, Twilight tries to soothe some of Ponyville's rampant paranoia by casting a spell to prove there are no imposters hiding in the town. But her spell leaves her in an alien body, facing a mob of angry ponies who just don't believe her when she says she is Twilight Sparkle.

Or, at least, she was Twilight Sparkle, anyway...

Featured on Equestria Daily, 2/17/13
Cover art is by the awesome and amazing Conicer