The article deals with an important factor in students’ study of the English language – students’ errors, which they commit in pronunciation, morphology, syntax and vocabulary due to the selection of the wrong phoneme, morpheme, structure or vocabulary item. It is stated that errors play an essential role in the investigation of language acquisition in general and in examining second and foreign language acquisition in particular. We are interested in errors as they are supposed to contain valuable information on the strategies that English teachers use to teach the subject. It is analyzed that errors are also associated with the efficient strategies that students employ to communicate in the English language, because they are believed to be a significant indicator of the students’ stages in their target language development. It is investigated that errors have a double purpose: diagnostic and prognostic. It is important that the notion of error is considered in order to clarify the phenomena that occur when errors are easily detected by language teachers, but are not always interpreted in the same way. The conclusion is made that the concept of error is not always objective as such factors should be considered in the study of errors as dialectal and contextual.