The principle
One sincere and honest move will cover over dozens of dishonest ones — a small honest confession buys cover for a much larger silence.
The application
A surprising vulnerability, named first, lowers the guard of the listener faster than any reassurance. Used well it is rapport. Used badly it is the longest con in the catalogue.
The defensive tell — how to spot it being run on you
When a new acquaintance opens with a small, charming confession of a flaw — especially one that flatters their character ("I work too hard," "I trust too easily") — the disclosure is a tool, not a gift. Take note and proceed warmly but normally; the confession bought trust it has not yet earned.