一失足成千古恨，再回首已百年身 yīshīzúchéngqiāngǔhènzàihuíshǒuyǐbǎiniánshēn a single slip brings eternal regret and by the time one looks back a lifetime has passed
a single misstep results in lifelong remorse, and by the time one realizes the mistake, one's life is already over; emphasizes the irreversible nature of serious mistakes
