To fail at the eleventh hour; to fall at the last hurdle; to lose success when it's almost achieved.
他们辛勤工作了几个月，眼看项目就要成功了，却因为一个意外的小失误而功堕垂成。
They had worked diligently for several months, and the project was almost successful, but an unexpected small mistake led to their efforts failing at the last moment.
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