Mistakes are things that are wrong in a learner's English. They can be divided into two types, errors and slips. Slips may not require feedback but errors will, at some point, or at least they tell us important things about the learner's knowledge.
A learner saying ‘I look forward seeing you' because they don't know that we use ‘to' in the structure is making an error, but if the learner says this because they are tired and not concentrating it may just be a slip.
In the classroom
There are many ways to give feedback on learner mistakes. Techniques for spoken mistakes include recording learners for delayed correction, and using gestures and echoing for immediate correction. Written mistakes can be addressed using correction symbols, group editing and models.