Incorrectly positioning ND grads
While ND grads are useful for seascapes, landscapes and other situations where there’s a clear boundary between the sky and the foreground, they’re less useful when a feature in the landscape protrudes into the sky. As the filter is lowered to darken the sky, there’s a risk that the feature will become darker too, so drawing attention to the use of the grad.
Using your camera’s depth of field preview can help you position the filter accurately, as the aperture will be ‘stopped down’, making the transition between the dark and clear parts of the filter more obvious. However, when you’re dealing with scenes that include buildings, lighthouses and cliffs, either use an ND grad with a soft transition, or do without the filter and bracket the exposure, so you can blend the pictures later in Photoshop.