Failing to understand responsibilities of the job. Don't take corporate job descriptions as gospel. Not only do they present glorified snapshots of jobs, they seldom accurately describe what the jobs entail. It's not all the companies' fault either. When running classified ads, companies are forced to reduce a complex job description to 50 words, which is impossible.
Many candidates never get beyond a headhunter's or HR person's description of a job.
You're setting yourself up for disappointment by believing what you read. It's the candidate's responsibility to dig deeper and find out what a job is all about. It's not only important for his or her own sake, it also helps make a great impression. Don't be uptight about politely asserting yourself and asking questions about a job. Get beyond the rosy four-line job description and find what you'll actually be doing, who you'll be reporting to, what your daily responsibilities will be, and what is the background and experience of your colleagues, etc.