What are some common red flags that can hurt an application?
In my experience the most common red flags are areas where students either neglected to include information, or entered information that was incorrect, or had one of their parents fill out the application. The former is a sign of sloppiness, the second one is a greater problem in and of itself.
When you are filling out an application, you should fill it out completely and accurately. Leaving questions blank can lead an admissions counselor to wonder why the question was not answered. Is there something the student is trying to hide? Doubt is not the first reaction you want a counselor to have when he or she is reading your application. You also want to make sure your information is accurate - even simple things like the name of your high school or your counselor's name should be spelled correctly. In fact, you should double check your entire application for spelling errors before you submit it - EVERYTHING should be spelled correctly.
When a parent fills out the application, any number of problems can come of it. Incorrect information, embellished information and if the application is done by hand, it can be quite apparent that a student did not fill it out him- or herself. This is irresponsible and unethical on everyone's part. This is also a situation where the college or university could choose to decline the application altogether regardless of the applicant's qualifications. Complete your applications yourself. If you want to have a parent help you with proof-reading, that's fine, but make sure you are the one answering the questions.
When an application is submitted with errors, it says that the student did not spend the time on it that it deserved. It can easily be assumed that the student rushed and did not take it seriously. When the application is filled out by someone other than the student, it says that the student is dishonest. Neither of these judgments are in the student's favor.