Such errors result from the learner's lack of knowledge of the correct rules of the target language variety.
The study of learners' errors has been the main area of investigation by linguists in the history of second-language acquisition research.
[2] In prescriptivist contexts, the terms "error" and "mistake" are also used to describe usages that are considered non-standard or otherwise discouraged normatively.
[8] H. Douglas Brown defines linguistic errors as "a noticeable deviation from the adult grammar of a native speaker, reflecting the interlanguage competence of the learner."
A distinction is always made between errors and mistakes where the former is defined as resulting from a learner's lack of proper grammatical knowledge, whilst the latter as a failure to use a known system correctly.