When I taught Native Spanish speakers AP Spanish in New York City, I found that they could speak clearly and communicate effectively in Spanish but their writing in Spanish was riddled with spelling errors.
This is a list of the 28 most common spelling errors (e.g. porque vs. por qué)I found in their writing. I used this list as a study guide and gave regular quizzes that tested their knowledge of these words.
If you like this list, check out the quiz that accompanies it. Titled "Common Errors Quiz".