mea culpa, mea culpa, mea maxima culpa
Through my fault, through my fault, through my most grievous fault.
I believe that is the correct translation of the Latin phrase. This is part of the confiteor, which in turn translates to I confess
The confiteor was used in the pre-Vatican II Roman Rite. It was repeated at the beginning of mass and on "various other occasions," according to the Catholic Encyclopedia