A man is lost in thought, somewhere in a public place. Perhaps he's standing on the street, or sitting on a park bench. His trip down memory lane has led the narrative into a sizable flashback sequence. What brings him out of his thoughts, and the plot out of the flashback? Why, a small boy tugging at his sleeve, piping the immortal line:
"Hey, Mister. Mister! You alright?"
This would possibly be categorized as a character-as-plot-device, aka "Characters As Device".