This example makes use of the Clone Stamp tool, together with Opacity settings and with Darken and Lighten Modes.  Sharpening an image often yields an unwanted artifact at the edge of the sky and a dark mountain.  In Fig. 1, a lighter line is at the base of the sky and there is a corresponding darker line at the top of the mountain.  Fig. 2 shows a blow up of this on the left side of the image, while the right side of Fig. 2 shows the result fixing the edge artifact.  The most common situation when this occurs is when your camera sharpens the image and then you sharpen it a second time in Photoshop.  It can usually be minimized if your camera does no sharpening at all, and then you control the sharpening selectively.
 

Fig. 1.  Edge artifact where the sky meets the land.

Fig. 2.  Blow up of Fig. 1 showing the edge artifact on the left side, and a fixed result on the right side.

The Clone Stamp tool essentially copies part of an image and pastes it in another part.  However, the tool becomes much more powerful when you change some of the defaults.  The Clone tool is applied directly to a layer, thus you will often need to flatten or merge layers when using it for this type of sharpening artifact.  We will first take care of the off-white edge in the sky, and then we will deal with the dark edge of the land in a similar manner.

Remove the off-white edge in the sky:

1. Zoom way in to the 300% or 400% level (or higher).
2. Click on the Clone Stamp Tool  in the toolbar.
3. In the menu bar, click on the brush size and change it to something small, probably in the 3 to 6 range for its radius.  As you move the mouse over the area to be changed, make sure that the circle is a little larger than the thickness of what needs to be fixed.  We will first fix the light (off white) edge in the sky.
4. For the Mode, change the value to Darken from the default of Normal.  This means that only things that change will be darker, and the result will be that the mountains will not change.
5. Change the Opacity level to 90%.  This minor decrease will yield a smoother result with the rest of the sky.
6. Right Click on the blue sky near where you plan to modify.  This is essentially the copy part of the copy and paste action.
7. Move the mouse over the white edge, press and hold down the left mouse button, and then trace out a short path over the white edge while holding down the left mouse button.  Do not attempt to trace out a long path for a couple reasons.  The color of the sky changes, so you should repeat the previous step to get a representative sample.  The second reason is a weird one.  Every now and then, weird things happen with the clone stamp, resulting in a dark ugly pasting.  With shorter paths, this is less likely to occur and if it does occur, you can Step Backwards and undo only a small amount of work.
8. Repeat each of the previous two steps 6 and 7 across the horizon.

Remove the dark edge from the land:

• This is almost identical to the previous steps for the sky.  The only differences are to change the Mode to Lighten (not Darken) in step 4, and to Right Click on the land (not the sky) near where you plan to modify in step 6.  Then trace out a short path over the large edge with the left mouse button down.