1. Using the Edit > Replace function
Don't use Microsoft Word's edit > replace function to replace the school name and try to use one school's essays for another school. Admission officers know each other's questions.
2. Not answering the essay question
Stanford's key essay is "What matters to you most AND why?" Make sure that you answer the 'why' part and not just the 'what matters to you most part'.
3. Embellishing
Don't use use vocabulary words that you've never used before in your essays. Also know that admissions officers do thorough background checks and will find out if you've exaggerated the truth too much.
4. Misspellings
Spell check is free. Use it! That may have been acceptable when typewriters were in style, but not in the year 2008. Whereas GMAT and GPA can be affected by a lot of external force (ie bad semester, sick on test day), essays are the portion of your application that you have the most control over and the most time to offer, therefore, they should be perfect.
5. Incomplete applications
Make sure that you answer every question. Though most application systems won't allow you submit an incomplete application, a lot of people don't read the questions correctly, therefore, they answer it incorrectly and thus the application is incomplete. An example is referencing your uploaded resume (ie "See my resume") on the online portion of the application when it specifically asks you to submit your top 3 achievements at your last job.
6. Underestimate the online application
All applications usually have 6 parts: GMAT scores, online application, essays, resume, transcripts, and letters of recommendation. The online application isn't just your name, address, and login information. It requires more time and thought than you think and is equally important to the rest of your application.
7. Submitting late
Don't even try it. Save yourself the fee and don't submit at all if you're going to submit late. (Exceptions are extremely rare.)
8. Leaving gaps
Don't leave the AO speculating. Fill in all the gaps in your story. They have great imaginations about 6 month gaps in resume. Use that additional essay to explain without writing another essay. Also, if you are not choosing to use your current supervisor as a recommender, please let the Admissions Committee know. It is fairly common that prospective MBAs do not tell their current supervisors about MBA pursuits, because it may jeopardize a bonus or a promotion. If you don’t choose your supervisor and you don’t let the AO know, they may assume that you have a bad relationship with your supervisor.
9. Playing defense
Don't worry about who else may be applying during your round. You can't play defense in the application process. The MBA application process in a one-person offensive sport like bowling, pole vaulting, and weighlifting. Just put your best foot forward.
10. Uploading the wrong essays