Lily and Severus
In spite of watching Lily, Severus is genuinely taken aback when she turns up and actually speaks to him, resulting in a moment of blank panic and wide-eyed staring in which he might be chewing his tongue. Maybe. But eventually he regroups, clears his throat, and ekes out a quiet, "Lily. Hullo." Pause. Duck. "Ah. You look well."
His social skills have clearly not improved since school.