Given that grammatical and spelling errors have been found to influence perceived competence and credibility in written communication, this study examined how a student's grammar and spelling errors affect how other students respond to the student's postings in four online debates hosted in asynchronous threaded discussions. Message-response exchanges were sequentially analyzed to identify patterns in students' replies to arguments and challenges with counter-challenges, explanations, and evidentiary support posted by students that exhibited low versus high number of grammatical and spelling errors. Although no causal inferences can be drawn from this study, the findings nevertheless suggests that: (a) arguments posted by high-error students are more likely to be challenged than arguments posted by low-error students; (b) exchanges between high-error students can amplify the effects of grammar/spelling errors; and (c) higher levels of argumentation can be achieved by placing students into groups that are heterogeneous in writing skills in general. The findings and methods used in this study lay the groundwork for further research on strategies for managing individual differences in students' grammar and spelling (and other student behaviors in general) and increasing the level of critical discourse in online discussions.