一失足成千古恨

a single slip may cause everlasting sorrow (idiom)

one / single / a (article) / as soon as / entire; whole; all; throughout / "one" radical in Chinese characters (Kangxi radical 1) / also pr. [yao1] for greater clarity when spelling out numbers digit by digit

to lose / to miss / to fail

to lose one's footing / to slip / to take a wrong step in life

excessive

foot / to be sufficient / ample

surname Cheng

to succeed / to finish / to complete / to accomplish / to become / to turn into / to be all right / OK! / one tenth

thousands

thousand

see 鞦韆|秋千[qiu1 qian1]

for all eternity / throughout all ages / eternity (used in an elegiac couplet, wreath etc dedicated to the dead)

surname Gu

ancient / old / paleo-

to hate / to regret