Use fault when explaining who is responsible for something bad.
It’s my fault that the car was stolen. I left the window open.
It’s my mistake the car was stolen. I left the window open.
Use mistake or error for talking about something that you did or thought which was wrong. Error is slightly
more formal than mistake.
We lost a week’s work due to a computer error.
I still make lots of mistakes in my essays.
I still make lots of faults in my essays.