She has made a grievous mistake.

她犯了一个极严重的错误。

youdao

Mr. Morris said the victims had suffered from a very grievous mistake.

莫里斯先生说受害者们为一个非常严重的失误而受苦。

《柯林斯英汉双解大词典》

Mr Morris said the victims had suffered from a very grievous mistake.

莫里斯先生说遇难者因为一个极其严重的失误而遭遇不幸。

youdao

The Center for Security Policy believes that, in light of the latest, powerful evidence of Mikhail Gorbachev's desire to preserve the power of the traditional central authorities in the USSR and his willingness to yield on this score only in the face of sustained pressure, it would be a grievous mistake for the West to come to his rescue.

CENTERFORSECURITYPOLICY: Center For Security Policy