The ambiguous antecedent strikes again. Where the noun that the pronoun takes the place of is not clearly defined. Classic examples -
Dog owners use powder to kill their fleas.
"I'm getting a drink, would you like one?" - "Sure" - "Great, I'll be back. Can I get you a drink while I'm at the bar?" (This one also falls into the double entendre category.)
I saw Esau sitting on a see-saw. How many S's in that? (none or five)
