College admissions officers aren't out to get you. Misplacing a comma or using "roll" instead of "role" aren't mistakes you want to make, but they're not going to get you rejected.
But some application mistakes glaringly point out that you're reusing an essay from another application or that you must not be all that interested in this particular school. Here are the three most common and embarrassing ones seniors must avoid.
1. Re-using an essay and forgetting to replace the school's name, as in writing in an essay to Northwestern, "Duke has everything I've been looking for in a college," or "That's why I can see myself spending four years in Cambridge" (where MIT is located) in an essay to Caltech (which is in sunny Pasadena, CA).
2. Mentioning an interest in a particular major that the school actually doesn't offer.
3. Re-using an essay that doesn't fully answer the question for the second (or third) college.
For example, Stanford asks you to describe a subject or idea that you find intellectually exciting and to explain why. A lot of students might try to re-use that essay for Cornell's supplement that asks, "Describe your intellectual interests, their evolution, and what makes them exciting to you. Tell us how you will utilize the academic programs in the College of Arts and Sciences to further explore your interests, intended major, or field of study."
If you write the Stanford essay and then just tack on a sentence at the end of it for Cornell like, "That's why I'm excited to study history at Cornell," it looks like what it is–an essay that you wrote for another college and are trying to pass off. Recycling an essay isn't necessarily always a bad thing; lots of colleges ask similar questions. But you don't want your essay to have an "insert name of college here" feeling.