A distinction is drawn between errors and mistakes. An error is when learners get language wrong because they don't know the correct form.

For example, a low-level learner might say 'If my mother didn't go to Spain, she not met my father.' because they have not yet learned the 3rd Conditional form (If my mother hadn't gone to Spain, she wouldn't have met my father). 
This student would not be able to produce the correct form even if they were told their sentence was wrong.

A student who has learned the form might still make a mistake with it, but does have the knowledge to self-correct, e.g., 'If my mother hadn't go to Spain, she wouldn't have meet my father.' In this case the teacher could isolate the mistakes and elicit the correct forms from the student.

Mistakes are sometimes also referred to as slips.

Mistakes and slips may also just be the kind of spoken and written mistakes native speakers make when tired or speaking quickly. 

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