Bowen and Marks, (1994) on mistakes
"Mistakes are not evidence of failure to learn; they are rather evidence of learning taking place."
Julian Edge (1989) on the 3 kinds of learner mistakes
Slips: mistakes the learner can self-correct
Errors: the learner can't self-correct, but involve a form the class may be familiar with.
Attempts: mistakes the learner makes when she has not yet learned the language necessary to express what she wants to say.