Here is an old image of a vulture against a gray sky.  In camera sharpening gave the halo.   Sure, this does not happen with every image but with some it is extreme, as in this case.  The second shot is the histogram of the shot, showing the strange lighting in action.

Note, the image is blown up a lot from the original.

This is one reason I do not use in-camera sharpening.   When sharpening, using any of the methods mentioned thusfar depending on the software you have, I recommend playing with the settings a bit and seeing the results. Extreme can mean extreme images...unsightly stuff.

I generally start with mimimal settings and work from there.  Now, I have a relatively set parameter for unsharp mask and also use high pass at no more than perhaps 28% fill.