a terrible mistake
It was a terrible mistake.
En la prueba cometí un terrible error.
I made a terrible mistake during the test.
He cometido un terrible error.
I have made a terrible mistake.
I made a terrible mistake.
Está claro que cometió un terrible error.
It's clear he made a terrible mistake.
There has been a terrible mistake.
El general John Pope cometió un terrible error.
General John Pope made a terrible mistake.