What are some common red flags that can hurt an application?
In terms of the application itself, failure to follow application directions can really hurt you. Sloppiness and mistakes that might indicate cutting-and-pasting between applications should also be avoided. With regard to your academics and background, periods of low grades, gaps in coursework, and very limited or too many extracurricular activities can be warning signs. Think critically about your application as you put it together. If there is something that might stand out and catch an admission officer's eye as a "red flag", acknowledge it and briefly address it either in your essay, the additional comments section, or via a reference letter from your counselor. Colleges are much more likely to move past a "red flag" if it's clear to them that you acknowledge it as such and are able to offer an explanation.