I forgot to add butter to my cupcakes but they were ok - why? I made some chocolate cupcakes and for some reason totally forgot to add the butter. The mixture was like muffin batter so I didn't twig until they'd cooked and looked  smaller then usual and more glossy on top.

On tasting them they were a little bitter, probably due to the amount of cocoa powder and more dense than usual but definitely not horrible, in fact quite nice.

Have I invented something or is this just something that I've actually made? I thought you had to have oil of some sort in cakes  to make it stay together?
Butter (and basically any saturated fat) is added to cake recipes mostly as a way of adding texture to the final product. By adding the fat product to the recipe, you make the cupcakes light and fluffy. If you leave the butter out, it will affect the taste slightly and will cause the cupcake to stay smaller and more dense, as you noted, but it will still be perfectly edible and probably quite tasty.