1. When he lost control of his emotions
Spock, half Vulcan, was always in control of his emotions. But in "The Naked Time," Spock got all kinds of intoxicated (thanks to Psi-2000 Polywater) and felt all of the emotions he'd been concealing. He cried. A lot. He regretted not telling his mother he loved her when he had the chance: a heartbreaking moment.
Watch Star Trek: The Original Series on CBS All Access.