You're thinking of "impeccability."
No, I was thinking of "infallibility", a notion among Catholics that the Church can not err. I guess that notion gets thrown on the ash heap of history.
In order to demonstrate that the Church acted fallibly, you must contradict the actual teaching of the Church regarding infallibility, not what you believe the Church teaches regarding infallibility.
Look through this Catholic Encyclopedia entry under Infallibility and cite a portion of the actual Church teaching.