He commanded his box and made some saves, then for the freekick he made a slight error of judgment and it cost us.

You are liable to commit a slight error of judgment that proves costly.

However, many endgames are forced wins only with flawless play, where even a slight error would produce a different result.

Any slight error, such as a brush of the wall, costs time.

But the very much alive painter was certainly much alarmed by this slight error.