Elaboration of well-considered classification of speech errors of multi-language schoolchildren is the necessary factor of successful providing Russian language programs in the situation of multicultural education. Such classification could allow differentiated approach towards the education and certification of multilingual pupils. The article is a study of different types of errors and the reasons of such innovations. The article describes common errors of Russian-speaking and multilingual children. Special errors of bilingual children (including those appeared as the results of interference) are also studied in the article. Errors caused by linguistic reasons and failures caused by particularities of processes of speech perception and generation are under consideration. While analyzing the linguistic nature of deviation, authors give examples of errors: connected with (1) the system violations (a person is not well acquainted with language rules); (2) violation of the norm (a person owns language rules but does not acquainted with constraints imposed by the norm); (3) violations of usus (a person is not acquainted with the conditions of realization of the language norm). Interlingual interference is supposed to be a cause of errors of the first type: authors list the examples of sintagmatic and paradigmatic interference; analyze the cases of improper segmentation of speech flow under the influence of native/first language and wrong differentiation of language elements on phonetic, grammar and lexical levels. The errors connected with violation of norm are studied as ones caused with the language system structure: they appear when there are absolute or relative lacunas in language. Facts that are controversial to modern language rules and cases when the means of units of high level does not come from the units of low level. The study allows to differentiate the errors caused by the input failure and genetic errors — the deviations caused by the processes of natural language learning that pass with the accumulation of pupils language experience.