How Darth Vader heartbreakingly met and remembered C-3PO, during The Empire Strikes Back.
After being blasted to pieces by a stormtrooper on Cloud City, C-3POs parts were brought to Darth Vader. Upon seeing the head of the droid he created, Vader was snapped into a joyful flashback of building him, but also a painful flashback of his childhood as a slave.
Darth Vader held his head against C-3PO’s for one final time, receiving one joyous memory of his mother, before showing a rare moment of compassion, and demanding the droid be returned to Chewbacca, without tampering with it’s memory or circuits.