"Woman has developed an imaginary, but useful, third arm":

After having a stroke, a 64-year-old woman reports that she now has a "pale, milky-white and translucent third arm" that she can use to scratch itchy parts of her body. She also says the limb can't penetrate solid objects.

...The woman underwent an MRI and when doctors askee her to move her imaginary third limb, her brain responded as if she really had the arm.

More details here.

Here's the abstract to the scientific paper and a media release.