Below is a great blog post from TheDesignCubical.com on the top 12 mistakes that anyone can make in Photoshop: http://www.thedesigncubicle.com/2009/01/12-common-photoshop-mistakes-misuses-and-abuses/

I know that I’ve fallen victim to at least one or 2 of these mistakes when I first started out. Photoshop can be a very tricky program for some people. I myself am married to Illustrator, and prefer to work in that program over any other.