Biff is presented to us as an All-American boy. As he grows up all Willy can do is criticize his son.
This construction implies that it is Willy who is doing the growing-up, rather than Biff. (The sentences need to be reworked:
"Biff is presented to us as an All-American boy. As he grows up, he receives nothing but criticism from Willy."
Ambiguity can also be produced by a mistake in vocabulary (many examples are dealt with in the Dictionary of Usage (45Kb) --see, for example, the entries on allusion/illusion and continual/continuous or word order (is a "big boys' school" a big school for boys or a school for big boys?).