As noted in their “20 Most Common Errors,” found on EasyWriter (Bedford St. Martins) and the FAME website, the following are the top 5.

Along with being the most common mistakes, they are also among the easiest mistakes to catch as you are editing your work. Remember, the best way to correct your own writing is to read it aloud.

5 Most Common Writing Mistakes

1. Missing comma after an introductory element: A missing comma after an introductory element can be heard as a pause as you are reading a sentence. For instance, read this sentence aloud and you will hear this element in structure.

2. Vague pronoun reference: A vague pronoun reference will leave the reader confused as to who is being discussed. For example, “The boy and the man fished as he drove the boat.”—who, exactly, is he?

3. Missing comma in a compound sentence (using and, but, so, yet, nor, or for): You want to be sure not to miss the comma separating the two (or more) separate ideas in a compound sentence. These parts of the sentence are joined by a coordinating conjunction (and, but, so, yet, or, nor or for).

4. Wrong word (using homonyms, words that sound alike, or words with a wrong meaning): Using a wrong word in a sentence may be hard to catch because the two words sound the same. For instance, “The girls put there letters in the mailbox.” There sounds right, but the correct word is actually their. Correcting this mistake takes a very close eye.

5. Missing comma(s) with a nonrestrictive element: Missing comma(s) with a nonrestrictive element can be caught as you are reading a sentence aloud. A nonrestrictive element is part of a sentence that is not necessary to its meaning. In the sentence, “The dog, who was very friendly, sat and waited for his dinner,” the nonrestrictive element tells us more about the dog.

For more common writing mistakes and explanations, visit the following websites:
http://www.oln.org/ILT/ada/Fame/cw/f4_48_463.html
http://bcs.bedfordstmartins.com/easywriter3e/20errors/