Implied pronouns--those that do not have an antecedent in the sentence nor in a preceding sentence--are difficult to spot in writing because they are so prevalent in our speech. Consider that all of the following sentences have implied pronoun errors:
They said on the news that pilot error caused the air show collision. [Incorrect]
I've been to the Smiths' house, so I should probably invite them to my party. [Incorrect]
Even though they said my grades were too low, I applied to Harvard anyway. [Incorrect]
We went to the hospital, but they said to just take two aspirin and call them in the morning. [Incorrect]
Do you see the problems in these sentences? Both the subject pronoun they and the object pronoun them are alarm bells on the ACT and SAT, so any instance in which these two words are underlined should make you pause and take a closer look.