I've seen plenty of cases where the writer forgets that it is a pronunciation thing and tosses in the an just because there is an initial vowel in the spelling of the following word. Then you get the case of someone choosing a or an based on the spelling of an initialism. Sometimes, when I am proofreading and come across a new (to me) initialism that I have never heard spoken, I have to ask an engineer if it is, e.g., a HID or an Aitch Eye Dee.