The acquisition of English as a second language at advanced levels presents unique challenges, particularly for learners in English for Specific Purposes contexts, where errors can impede academic and professional communication. This study examined persistent errors in the written and spoken English of advanced Iraqi ESP students in STEM fields, focusing on three interrelated factors: language interference (interlingual and intralingual), socio-cultural norms influencing discourse patterns, and the absence of code-switching as a significant variable. A mixed-methods design employed 100 MA (n=50) and PhD (n=50) STEM students from Iraqi universities, screened for advanced proficiency via the Oxford Quick Placement Test (scores 55-60). Data collection included 300 academic essays, semi-structured interviews, focus group discussions, and surveys. Analysis integrated quantitative error frequency counts using Surface Strategy Taxonomy (Dulay et al., 1982) and Linguistic Category Taxonomy (James, 1998), alongside qualitative thematic analysis, pragmatic frameworks (Grice, 1975; Leech, 1983), and sociolinguistic approaches (Hymes, 1974; Labov, 1972). The findings revealed that language interference, specifically interlingual transfer from Arabic—dominated written errors, accounting for approximately 80% of all errors for both MA and PhD students, particularly in article usage (16.8%), preposition selection (14.5%), and word order (11.4%). Intralingual errors (20%) were more prevalent in verb tense/aspect, reflecting the inherent complexity of English grammar. Notably, the study found that code-switching was not a significant factor in participants’ language production, with minimal instances observed across the corpus, indicating that advanced learners maintain strict language boundaries and do not rely on L1 to compensate for linguistic gaps. In contrast, socio-cultural norms exerted a powerful influence on spoken discourse, with indirectness/hedging (87% prevalence) and elaborate politeness formulae (82%) strongly correlated with Iraqi high-context communication norms, leading to pragmatic mismatches in international academic settings. The perceived impact of these errors was substantial: 92% of participants reported reduced clarity and misunderstanding, 85% experienced negative perceptions of their competence and credibility, 78% described communication anxiety and avoidance, and 72% faced challenges in collaboration and networking. These findings underscore the need for targeted ESP instruction addressing interlingual interference through contrastive analysis, developing pragmatic awareness to mitigate socio-cultural mismatches, and recognizing that code-switching plays no significant role at advanced proficiency levels, thereby requiring different pedagogical strategies than those employed for intermediate learners.
