The purpose of this study was to determine the most common grammatical errors found in students' theses at a private university in Southeast Sulawesi, and to consider the possible causes of the students' mistakes. Descriptive qualitative methods were used. Students were recruited through purposive sampling. The theses were analyzed using the Mileys & Huberman's approach. During the data collection process, the writer documented the thesis background and noted any errors that were discovered.

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... errors were found in the background of students' theses. The most common errors were on misformation, which accounted for 547 or 68.12% of all errors, followed by omission errors, which accounted for 128 or 15.94% of all errors. Misordering and addition, on the other hand, accounted for 11.83% and 4.10% of all errors, respectively. The researchers deduced from the error analysis that mother tongue interference and target language causes contributed to these errors (false analogy, misanalysis, incomplete rule application, exploiting redundancy, overlooking co-occurrence restrictions, hypercorrection, overgeneralization, or system-simplification). Keywords: frequent grammatical errors, misformation, misordering, addition, omission