mistake
‘mistake’ and ‘error’
A mistake is something incorrect or unfortunate that someone does. You say that someone makes a mistake.
He made a terrible mistake.
We made the mistake of leaving our bedroom window open.
In more formal English, you can use error with the same meaning. You also say that someone makes an error.
The letter contained several spelling errors.
He made a serious error in sending the man to prison.
Be careful
Don't say that someone ‘does’ a mistake or an error. Don't say, for example, ‘He did a terrible mistake’.
Don't say that someone ‘does’ a mistake or an error. Don't say, for example, ‘He did a terrible mistake’.
You say that someone does something by mistake or, in more formal English, in error. Don't say ‘in mistake or `by error’.
I went into the wrong room by mistake.
She was given another student's report in error.
‘fault’
Don't use ‘mistake’ or ‘error’ to refer to something wrong in a machine or system. Use fault.
The machine has developed a fault.
I tried to call him on the phone, but there was some sort of fault on the line.