Imagine receiving a notice in the mail that your application to a particular college is incomplete due to a missing item like test scores or a letter of recommendation. Suddenly, this whole application process doesn't look quite as finished as you thought it was.
But if this happens, don't panic. Admissions offices receive thousands of pieces of mail during application season and each one needs to be individually sorted and filed. The occasional missing item is a normal part of the process even for kids who did everything right. That's why colleges build in time to alert those students to re-send the missing information.
If you get that kind of notice, no need to panic. You didn't necessarily do anything wrong. Just (re)send the requested information as soon as possible.