Mea culpa, mea culpa, mea maxima culpa.
Translated from Latin as "Through my fault, through my fault, through my most grievous fault."
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 during the consecration, specifically when the celebrant raises the host before the congregation.
Old school Catholics will still repeat under their breath this during Mass, typically while hunched forward and lightly beating the breast with their right hand. Young Catholics don't do this unless they wish to appear pious; they're much more likely to say "my bad" when fumbling on a field at recess.