Definition : An error is an inaccuracy in speech or writing caused by the learner's lack of knowledge of the required form.
Example : the student who says "I live here for three years" before s/he has encountered the present perfect.
Errors contrast with mistakes which are slips caused by eg tiredness, anxiety or the pressure of the communicative situation. The learner knows the correct form and can self-correct if s/he recognises the mistake or if it is pointed out.
Bartram, M. and Walton, R. Correction: A Positive Approach to Language Mistakes, Heinle ELT