serious mistake
He descubierto un grave error.
I have discovered a serious error.
It would be a serious mistake to do so.
Oye amigo, cometiste un grave error.
Hey, buddy, you made a serious mistake.
— मुन्ना भाई एम बी बी एस (2003)
Cometí un grave error al elegir a mi esposa.
I made a grave mistake in choosing my wife.
She made a serious mistake.