Effectively leading whole-class mathematics discussions is made more difficult when students' contributions are incomplete, imprecise, or not yet correct, and not easily correctable--what we call "errors." Through purposefully designed opportunities to investigate, enact, and reflect on teaching, teacher candidates (TCs) can develop skill to productively respond to errors in whole-class discussions. We investigate how TCs respond to errors when engaging in a written performance task that calls for TCs to play out discussions in response to a classroom scenario. We consider what the performance task reveals about TCs' practice and perspectives, with implications regarding theory and practice. [For the complete proceedings, see ED606531.]