The most common error I see when editing is pronoun agreement. People jumble up singular pronouns (his, her, or its) with plural ones (their). Make sure that you are consistent. If you use singular, make sure all references are singular. The same is true for plural. Many folks don't want to use his/her, so they make all references plural. That's fine. You just need to be consistent.

Incorrect Examples:
Either Heather or Jocelyn will have to give up their office. (Their should be her.)
If anyone wants a cookie, they should ask. (Anyone and they don't match. You'll need to change one of them.)

Correct but Clumsy:
The parent of teens often wonders where he or she went wrong with his or her decisions. (This is correct, but I would make it plural to eliminate all of the he/shes and his/hers.) (The parents of teens often wonder where they went wrong with their decisions.)