First of all, it is common in ELT to establish a difference between an error and a mistake.
An error is when a learner uses language incorrectly because they are a) not aware of the correct form or b) unable to use the correct form or c) they have misunderstood how the form is used.
In other words, errors tell us that our learners are not able to use the form correctly.
A mistake is when a learner uses language incorrectly because they are a) tired and under pressure to perform in real-time communication (native speakers often make these mistakes too) or b) able to use the correct form but have used the incorrect form due to carelessness or lack of attention.
In other words, mistakes tell us that the learners are just like me or you and are not perfect
This leads us to the key question: How do we know when a learner has made a mistake or an error?
One technique we can use is to see if the incorrect language can be corrected.
Firstly, see if the learner who made the mistake can produce the correct form – SELF-CORRECTION
Secondly, see if another learner can produce the correct form – PEER-CORRECTION
Finally, if nobody can produce the correct form, you will probably have to correct- TEACHER-CORRECTION
If the learner who made the mistake can self correct, they probably made a mistake.
If a peer needs to correct, the learner who produced the incorrect form might have made a mistake or an error. We should see if they can now produce the correct form.
If nobody can correct, this might mean that the learners are not aware of the form or use it erroneously. You might try to teach then the correct form on the spot or decide to come back to it at a later time.