Is over-writing one of the most common mistakes an applicant can make on his essay?

I would not call rewriting a mistake. It is rather a gross violation. There are many common mistakes that students make in their written work. Here are some of them:

1. The substitution of one genre of written expression for another, namely, the composition "My Opinion" is replaced by the composition "Pros and Cons".
2. Departure from the topic.
3. Inconsistency of style with a written statement with elements of reasoning.
4. Inconsistency of the argument with the stated thesis (opinion).
5. Repeat of argumentation in expressing one's and others' opinions.
6. Lack of detailed argumentation.
7. Incorrect formation of counterarguments.
8. Inability to formulate the problem at the beginning of the statement (simply repeating the wording of the assignment) and conclude at the end.
9. Incorrect paragraphing.
10. Logical errors, including the discrepancy of the author’s point of view in the second paragraph and conclusion.
11. Failure to use or misuse of logical communications
12. Lexical and grammatical and spelling errors.
13 Non-compliance with the required volume.

In order to avoid mistakes, you should read about the structure of the essay, broaden your horizons as a whole, and train more. You can also ask for help from a teacher, private tutor or in special services, for example check that. Good luck!