Unless it was something really obvious like mispronouncing a character's name or something that is really part of canon (like, I remember listening to this one SGA fic by a British podficer (I can't remember who), where they pronounced lieutenant the non-American way with the F sound in the middle. Even though that's a legit way to say that word, it's not what John is, he's in the American military).

For me, it only bothered me when John or another American character said it that way in dialogue, not when the narrator used it. (I don't think it would have bothered me if the reader had simply replaced one vowel for another, i.e. "tomahto" for tomato or "conTRAHversy" for controversy. But when you start playing with consonants to me, they become heteronyms.)