This study examined the interaction of different types of crosslinguistic cues in second language (L2) morphosyntactic processing. Our target constructions, Korean morphological causatives, contain morphosyntactic constraints that present interlingual overlap for Japanese speakers when the construction is derived from an intransitive verb, while constituting interlingual contrast when derived from a transitive verb. For Chinese speakers, these constraints only exist in the L2 and thus constitute L2-unique information. In two self-paced reading experiments involving proficiency-matched Japanese- and Chinese-speaking learners of Korean, we found that Japanese speakers successfully detected morphosyntactic errors only in the intransitive-based construction, which shares overlapping constraints with Japanese, but not in the transitive-based construction whose morphosyntactic constraints contrast with the Japanese counterparts. In contrast, Chinese speakers exhibited sensitivity to the violations in both intransitive- and transitive-based constructions. These findings suggest that crosslinguistic competition causes a major problem in L2 sentence processing.