There have been some word modifications over the years, e.g., for years Blatty had misattributed “God is love” to St. Paul, whereas it's from John's Gospel – newer editions corrected that.
One grammatical error always bothered me: when Karras finds a natural explanation for one of Regan's manifestations, Blatty notes Karras' disappointment: “…as he slowly watched wine turn back into water”. Actually Karras wasn't watching “slowly”. Blatty means to say that the wine is slowly turning back into water. So it should read, “…as he watched wine turn slowly back into water”. Small gripe, I hope it's fixed in newer editions.