Confusion is particularly prevelant when you are writing about two people of the same sex, in which case you often end up with sentences like this one:
She told her that her face was dirty, and she told her to take a bath.
Another pronoun problem is the indeterminate "this":
Because of this, I can't go to the moon.
The reader will ask, "This what?" When you use a pronoun, make sure that its meaning is absolutely clear.
For more information, see the pages about subject/verb agreement, pronoun agreement, and the general discussion on agreement.