Introduction In the process of learning a second language (L2), errors are inevitable. The discipline of error analysis was developed in the 1960s, and since then, the study in the field was geared towards understanding the learning process difficulties (Zheng & Park, 2013, p. 1342). The results are being used to improve teaching practices, but it should be pointed out that the most developed area of error study is concerned with grammar. The research of lexical errors is sparse, and the study of lexical errors in the writing of EFL/ESL students is particularly underdeveloped (Shalaby, Yahya, El-Komi, 2009). This issue is not caused by neglect or by the insignificance of the field of study. On the contrary, the importance of vocabulary for language learning is not being questioned nowadays (Hemchua & Schmitt, 2006; Llach, 2011). Vocabulary is a significant indicator of the level of learners’ proficiency since it is correlated with their communication abilities (Llach, 2006). The lack of systematic rules for lexical items makes the study in the field more difficult and complicates the acquisition of vocabulary (Kaweera, 2013). As a result, the study of lexical errors and lexical errors in writing is an essential but underdeveloped and underresearched topic. In this paper, the scientific background for lexical error study is provided. Apart from that, the phenomenon of language interference is defined as a part of language learning and acquisition, and its impact on lexical errors is explained. Then, the notions of error, lexicon, lexical error, and lexical unit are described, and generalized taxonomies for lexical errors are presented. Finally, the issue of lexical error in writing is discussed, and the implications of lexical error studies are considered. It is concluded that the study of lexical errors in writing is capable of promoting our understanding of the theory and practice of language learning and acquisition. Lexical Errors in Writing: Scientific Background Lexical errors have received less coverage in scientific writing than grammar mistakes, even though the vocabulary is a crucial part of a student’s knowledge of a language (Llach, 2011, p. 70). Even less research has been carried out with respect to lexical errors in writing (Hemchua & Schmitt, 2006). The problem consists in the lack of constant rules in the area: the lexical system and its elements are much less capable of a generalization than, for example, morphology or syntax. Consequently, the errors in this area are less generalizable as well; apart from that, they often depend on context, which makes them even more unique. The interloping of the areas of grammatical and lexical studies (for example, the issue of grammatical words) has also been a complicating factor. As a result, the topic of lexical errors in writing remains underdeveloped, which invites further research in this area. Basic Aspects and Terminology Llach (2011) points out the ambiguity of the terminology that is related to the issue of lexical errors. Here, several definitions and basic notions are outlined. Language Learning and Acquisition Usually, two types of non-native language (L2) learning are distinguished: studying a language as a foreign one and studying it as a second one. For the English language, the abbreviations of such approaches are EFL (English as a Foreign Language) and ESL (English as a Second Language). The differences between EFL and ESL teaching and studying a