This study aimed to find out the most common grammatical errors made by the eleventh-grade students of XI IPA 1 at SMAN 1 Panyabungan Selatan in writing recount texts and to describe the sources of the errors. This research employed a descriptive qualitative method. The data were obtained from students’ recount texts and semi-structured interviews. The students’ grammatical errors were analyzed based on Dulay, Burt, and Krashen’s Surface Strategy Taxonomy, which consists of omission, addition, misformation, and misordering. The interview data were analyzed to identify the sources of students’ errors, including interlingual transfer, intralingual transfer, context of learning, and communication strategies. The findings showed that there were 300 grammatical error occurrences found in the students’ recount texts. The errors consisted of 142 misformation errors (47.33%), 92 omission errors (30.67%), 38 addition errors (12.67%), and 28 misordering errors (9.33%). Therefore, misformation was found to be the most common grammatical error made by the students. The grammatical problems were mainly related to the use of simple past tense, verb forms, auxiliary verbs, subject–verb agreement, articles, plural markers, pronouns, prepositions, and word order. The interview findings indicated that the sources of students’ grammatical errors included interlingual transfer, intralingual transfer, context of learning, and communication strategies. Among these sources, interlingual transfer was the most frequently identified, indicating that students’ first-language and Indonesian language structures influenced their English grammatical production. In conclusion, the students of XI IPA 1 experienced various grammatical difficulties in writing recount texts, with misformation being the dominant type of error and interlingual transfer being the most prominent source of errors. These findings are expected to provide useful information for English teachers in identifying students’ grammatical difficulties and developing appropriate instruction to improve students’ accuracy in writing recount texts.