Why is this ambiguous?

Hi Everyone,

Recently a linguistic teacher in our department came to see me. She was wondering why something she had a read in a book was considered to be ambiguous. At first, I was as confused as she was but after a few minutes thought, I figured out why the book said it was ambiguous. What I would like you to do, if you want to, is to figure out what makes the passage below ambiguous. After I get some posts, I will give the answer which I gave to your teacher. Good luck, Mark

"An old man's bike."