Errors generate typical brain responses, characterized by two successive event-related potentials (ERP) following incorrect action: the error-related negativity (ERN) and the positivity error (Pe). However, it is unclear whether these error-related responses are sensitive to the magnitude of the error, or instead show all-or-none effects. We studied error-monitoring with ERPs while healthy adult participants performed ballistic pointing movements towards a visual target with or without optical prisms, in alternating runs. This allowed us to record variable pointing errors, ranging from slight to large deviations relative to the visual target. Behavioural results demonstrated a classic effect of prisms on pointing (i.e. initial shifts away from targets, with rapidly improving performance), as well as robust prismatic after-effects (i.e. deviations in the opposite direction when removing the prisms after successful adaptation). Critically, the amplitude of both ERN and Pe were strongly influenced by the magnitude of errors. Error-related ERPs were observed for large deviations, but their amplitudes decreased monotonically when pointing accuracy increased, revealing a parametric modulation of monitoring systems as a function of the severity of errors. These results indicate that early error detection mechanisms do not represent failures in an all-or-none manner, but rather encode the degree of mismatch between the actual and expected motor outcome, providing a flexible cognitive control process that can discriminate between different degrees of mismatch between intentions and outcomes. (Contains 4 figures.)