Fallacy of the Selfless Act

Wisdom from the "Dialogue of St. Catherine of Siena"

"This is the supreme office of discretion which proceeds from
charity. So you see how discreetly every soul, who wishes for
grace, should pay her debts, that is, should love Me with an infinite
love and without measure, but her neighbor with measure, with a
restricted love, as I have said, not doing herself the injury of sin in
order to be useful to others. This is St. Paul's counsel to you when
he says that charity ought to be concerned first with self, otherwise
it will never be of perfect utility to others. Because, when
perfection is not in the soul, everything which the soul does for
itself and for others is imperfect. It would not, therefore, be just
that creatures, who are finite and created by Me, should be saved
through offense done to Me, who am the Infinite Good. The more
serious the fault is in such a case, the less fruit will the action
produce; therefore, in no way should you ever incur the guilt of
sin.

www.catholicspiritualdirection.org...

I know I'm a little late to the thread but I just found it. I would like to submit this act to the discussion. I think it meets the definition the OP was looking for as a truly selfless act.

Pa. Soldier Who Jumped on Grenade to Receive Posthumous Medal of Honor

Given the situation and the extreme short period of time of the events I do not think the person had any time to think about anything other than to save the lives of his fellow soldiers.