The dog chased the cat, and it ran up a tree.
Problem: It's unclear what "it" refers to. Did the dog run up a tree, or did the cat?
Solution: Be more specific:
* The dog chased the cat, and the cat ran up a tree.
* The dog chased the cat, and it (the cat) ran up a tree.
Other examples of unclear pronoun reference:
* John told his friend that he was going to the store. (Who is "he" referring to? John or his friend?)
* I put the cookies on the table, but they were gone. (Were the cookies gone, or was something else gone?)
Key takeaway: Make sure your pronouns clearly refer to their intended antecedents (the nouns they stand in for) to avoid confusion.