Error
A mistake that a learner makes when trying to say or write something above their level of language or language processing.
A developmental error is an error made by a second language learner which could also be made by a child learning their mother tongue as part of their normal development. A second language learner might make the error because they are applying a rule they have learned that doesn’t work for this particular case e.g. I goed there last week (I went there last week).
A fossilised error is an error that has become (almost) permanent in a learner’s language and has become a habit. Fossilised errors cannot easily be corrected. For example, a B2 learner might habitually not add an ‘s’ when saying third person singular present simple verbs. Learners at this level do not usually make this mistake, but, for this learner, the error was not corrected early and it has become habitual. See fossilisation.
A slip. When a learner makes a slip they make a language mistake but they are able to correct themselves, e.g. Learner: He like ice-cream, I mean, he likes ice-cream.