People are constantly spoiling a project when it lacks only a step to completion.

Lao-tzu (604?-531? BC) Chinese philosopher, poet [also Lao-tse, Laozi]

The Way of Life, 64 [tr. Blakney (1955)]

Variant (?): "People in their handling of affairs often fail when they are about to succeed. If one remains as careful at the end as he was at the beginning, there will be no failure."