Abstract: Language learners may use an unacceptable combination of words and sentence structures, leading to what is perceived as errors. For example, a Spanish learner might say La estación de tren es en esta calle, instead of La estación de tren está en esta calle. I investigate how and why language learners have trouble distinguishing unacceptable sentences from acceptable ones and whether exposure alone can make language learners detect errors. I find that learning the unacceptability of sentence structures is particularly difficult for language learners (Tachihara & Goldberg, 2020). However, I also find that repeated exposures over multiple days lead Spanish learners taking undergraduate classes to identify unacceptable sentences as unacceptable, showing that competition is a key component of learning distinctions. This effect was linked to the proficiency of the participants, revealing that the more proficient they are, the more likely they are to identify unacceptable sentences as unacceptable. In other words, language learners are learning what is wrong in a language. Finally, I discuss why learning what is wrong in a less dominant language is difficult by offering a cognitive explanation. I hypothesize that between-language competition (i.e., competition between one’s first and second language) affects within-language competition (i.e., competition between acceptable and unacceptable sentences). This makes competition less effective at reducing the acceptability of unacceptable sentences, requiring extensive exposure to reach the same effect.