This study aims to explore how the markedness effect shapes Mandarin slips of the tongue with respect to nasals in syllable-final positions. Data were collected via natural speech and elicitation tasks from 35 participants' reading of 346 test items. Three hundred and eight slips in Mandarin from natural data and 360 slips from elicited data were investigated. This study shows that there exists a strong preference for unmarked coronal nasals over marked dorsal nasals in the syllable-final position in both spontaneous and elicited Mandarin slips. This tendency toward the unmarked [n] reveals that the influence of the unmarkedness effect is present in the coda position in Mandarin slips. Interestingly, this tendency is inconsistent with that found in a previous study by Wan of Mandarin slips in the onset position, which shows a tendency for coronals to be replaced by other sounds. This suggests an onset--coda distinction occurring in Mandarin speech errors, which implies that the markedness effect works differently in distinct syllable positions. More precisely, the unmarkedness effect emerges in the coda position in Mandarin slips, leading to the occurrence of unmarked segments, whereas it submerges in the onset position, resulting in marked segments. (Contains 1 figure, 16 tables and 25 notes.)