Forgetting to fix lens problems
Lens distortion can create a myriad of problems, so make this the first adjustment you apply when you process an image. If you don’t fix distortion, then images may not appear as sharp; lines that should appear straight, such as the horizon, will appear to bow; and the corners of an image may be darker than its centre.
You can apply some lens corrections in-camera, such as using Vignette Control to reduce the amount of corner-shading a lens exhibits (typically a problem with fast lenses), but carrying out the adjustments post-shoot gives you more control. If you do plan to apply lens corrections, then compose wider than normal, particularly with a wide-angle lens, as the stretching and cropping that occurs can slice a surprising amount from the edge of the picture.
Chromatic aberration (colour fringing) can be obvious along edges in high-contrast shots, but it’s easy to remove automatically in Lightroom and Photoshop, which have lens profiles already built in