This is the last chord of Francis Poulenc's Mélancolie for piano. It occurs while a piano D♭ major chord is still resonating in the bass.
How would you have the B♭♭ be perceptible, surprising, and yet still pianissimo? Any technique?
I tend to think that any single performer would judge himself negatively if this note did not sound properly, and I feel that I miss it approx. 30% of the time.