One might argue that a spelling mistake must not be that big of an issue in a thesis but for the review committee, it is! Right from the start of documenting your research, you must include proofreading, at least once, in your timetable.
Things you need to check:-
1. Check if all factual data is cited appropriately.
2. Check if all sources are properly cited.
3. Remember to check each and every word of your thesis for spellings as well as capitalization.
4. Check your document for consistency.
5. Review the grammar in the document.
6. Pay attention to logic, flow and transition in the document.
7. Make sure all figures, tables and graphs are properly labelled.
8. Read thoroughly to ascertain if the thesis communicates all that it intends to, and in an effective manner.
9. Check if you have answered all the research questions adequately.
10. Check the chapterization of the thesis.
11. Check if references are in the prescribed format.
You may even get your document proofread from a friend, a colleague or a professional editor. Irrespective of how you get it done, proofreading your thesis before submission is a must!