If you haven’t seen this before check it out, this article is by Brian Hoff (@behoff) on The Design Cubicle:

12 Common Photoshop Mistakes and Malpractice

Excerpt from article: -

12 common mistakes, misuses and various ways new users abuse Photoshop. I have fell victim to a few of these myself when first starting out, so I hope this article helps educate designers and other individuals new to Photoshop.