Not Considering Location, Location, Location: Jobs in certain occupations are concentrated in specific cities—Dublin or London for example—or in certain types of locations—such as cities versus rural areas. If you live somewhere that doesn't offer many opportunities in your field and you aren't willing to relocate, you will have trouble getting a job.
Not Looking Beyond a "Best Careers" List: Lists that tell you what careers have the best opportunities of the year, decade or whatever, can be a helpful guide when it comes to selecting a career. However, making a decision based solely on one of those lists is a terrible idea. Even an occupation with a great outlook can be a bad fit, so you have to scratch below the surface to find out whether you and a career are a good match.