“Don’t say that as a boy your grandmother used to read to you, unless at that stage of her life she really was a boy, in which case you have probably thrown away a better intro.”
I am currently reading The Elements of Style, by Strunk and White...
Rule 7. A participial phrase at the beginning of a sentence must refer to the grammatical subject. Participial phrases preceded by a conjunction or by a preposition, nouns in apposition, adjectives, and adjective phrases come under the same rule if they begin a sentence.
(HT: Justin Taylor)