1. Leaving any section of the application blank

2. Not sharing activity details

3. Expecting your stats to do all the talking

Here is what Caltech says on this point:

Applicants should take special care to address how they will take advantage of the college's resources to develop themselves both in the classroom and in the community.... Admissions committees cannot derive your potential community impact from GPAs or class ranks, so applicants must make cases for themselves as people as well as students.