I'd guess the "Under God" addition that came much later. I didn't know the whole "Under God" addition was spearheaded by the Knights of Columbus, though.
Ever since I was about, oh, 11ish, I stopped reciting the Pledge of Allegiance (though I did stand respectfully). I've always been a bit of a rebel, and while I enjoy living in this country, I don't feel the country itself (nor its flag) should be subject to a pledge of allegiance engrained into children too young to understand what they're pledging. Perhaps I was just a dumb kid, but until then it was just a thing you mouthed out in a drone at the beginning of class - a ritual, not a solumn vow, as it should be.
Better no pledge at all until the full extent of what is being pledged is understood by those stating it, in my humble opinion.