In flanker tasks, heterogenous-symmetric letter-combinations (e.g., HXH) typically indicate decreased behavioral performance when compared to homogeneous combinations (e.g., HHH) or heterogenous-asymmetric combinations (e.g., HHX; Donohue et al., 2016). Despite of the stimulus conflict being task-irrelevant, this performance decline was even stronger when presenting the identical letter combinations in a symmetry judgment task instead of a flanker task (Kähler et al., 2020). Thus, symmetric judgements might be deteriorated by an underlying conflict interference. Using the ERP-components Ninc as an indicator for cognitive conflict and SPN as an indicator for symmetry evaluation, we tested for neuronal evidence of conflict processing during symmetry judgments. Participants performed both tasks in separate blocks, order counterbalanced. In both tasks, behavioral reaction-times were fastest to homogeneous stimuli compared to all other stimulus conditions, and even more pronounced during the symmetry than flanker task. In contrast, error-rates only differed in the symmetry task by indicating more errors for the heterogenous-symmetric condition compared to all other conditions. In the ERP-data, Ninc-activity was increased, independently of tasks, when comparing heterogenous-symmetric and heterogenous-asymmetric conditions to the homogenous condition. Further, in the symmetry task, an early SPN-activity (250-300ms) was pronounced for the homogenous condition, switching however to a late SPN-activity (350-500ms) with a remarkable negativity increase for heterogenous-symmetric condition. Thus, while conflict-related Ninc-activation seemed to mirror reaction-time differences, late symmetry SPN-activity seemed correlated to error-rates. In conclusion, processing of heterogenous-symmetric letter-combinations seems to lead to cognitive conflict before symmetry evaluation, even when conflict between letters is irrelevant to the current symmetry judgement.