From HBR: To Collaborate More, Try an Olive Branch on Steroids
With the exception of “dyed-in-the-wool” unforgiving types (you know, the ones who seem to delight in ruining family holiday dinners), one of the things that nearly the rest of all other people are defenseless against is a sincere, earnest, unsolicited apology.
In fact, there are not a small number of people in this world who have never received one. And I mean never. But even for the majority of people, it is a rare enough occurrence to cause them to be disarmed and intrigued and to lower their guard to hear what the apologizer has to say.