Pet peeve of mine, but I don't think this is a purposeful grammatical error (as some NPCs with accents / ways of speaking have).

In Western La Noscea for the quest "Scythe Extra Large" when turning in the quest he says: "I'm sure Aerghaemr wouldn't of wanted anyone dying near his resting place."

Needless to say, it's "wouldn't have" not "wouldn't of".