功败垂成
A slip betwixt the cup and the lip.
A flash in the pan.

eg: Tom is disqualified at the final round, a slip betwixt the cup and the lip.

中英俚语俗语来源：汉书：“垂成之功，败于一日。”

同义词：功亏一篑 Look back from the plough.
反义词：一蹦而就 At one stroke.