Whenever stacked objects have the exact same shape, or when using a clipping mask, the colors from the objects below will be visible around the edge of the upper object. See screenshot.

This will probably not show when you print, but it does show when you rasterize a vector image. The screenshot is from a rasterized .ai image opened in Photoshop. So it's not just a preview bug in Illustrator.

And it's quite disturbing to see these ugly edges when work on your illustration.