Sometimes, even under the best of circumstances, you find an error in your applications after you've submitted them.
We know you're human. So do admissions officers. Your human capacity for error doesn't give you a pass to submit a mistake-riddled application, and ideally, you've submitted applications with zero mistakes. But if you do find a mistake after you've already sent them off, don't panic. There are things you can do to set things right again:
Not every mistake is a doozie. If the mistake is truly inconsequential (one typo in the name of your swim club), let that sleeping dog lie rather than drawing attention to the mistake with a correction. But if the mistake is anything bigger than a single typo, submit a corrected version of your application form (or the attachment only, if the mistake was in an attachment) with a brief cover letter asking that your corrected application/attachment be substituted for the previous one.
Submitting a corrected application can be logistically tricky in the era of online applications, because once your application has been submitted, you are typically prohibited from changing it or resubmitting it. If the college is a Common Application college, you should submit your corrected version and your cover letter directly to that college. You cannot submit any updates or corrections through the Common Application after you have submitted.
There's no harm in redundancy in this case. Make sure to keep both the affected colleges and your high school counselor in the loop. Your counselor might even be able to facilitate whatever correction you need to make.
On the Common Application, if your mistake appears on any component other than the essay, you can correct it without creating a new version of the Common Application. If, however, the mistake was in your essay, then you will need to create an alternate version of your essay in the Common Application system. Because this alternate version will use up one of the three alternate versions that you are allowed to use, make sure that you are comfortable using one of your alternates for this purpose. If you are not, check with the college to see if you can submit a corrected version of the essay directly to that college.