The APA Style blog (2013) listed the top 10 APA mistakes. I would definitely agree these are the top mistakes I see when editing a dissertation. Please know, the university editors flag these as well. Other mistakes I would add, although not technically considered APA mistakes, are missing references and non-matching references, i.e., Janes, 2012 in the reference list and Jones, 2012 in the text.
1. Incorrect use of numbers
2. Incorrect use of hyphenation
3. Incorrect use of et al.
4. Incorrect capitalization and punctuation in headings
5. Use of since instead of because
6. Improperly prepared tables and figures
7. Failure to use the serial comma
8. Failure to spell out abbreviations and acronyms as needed
9. Inconsistent use of double-spacing between lines
10. Incorrect use of and versus the ampersand
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