a terrible mistake
It was a terrible mistake.
Está claro que cometió un terrible error.
It's clear he made a terrible mistake.
¡Araya, cometiste un terrible error!
Araya, you made a terrible mistake!
— El jardín de los pecadores 5: Paradoja espiral (2008)
There has been a terrible mistake.
El general John Pope cometió un terrible error.
General John Pope made a terrible mistake.
He cometido un terrible error.
I have made a terrible mistake.