The present study tested error detection theories of the error-related negativity (ERN) by investigating the relation between ERN amplitude and error detectability. To this end, ERN amplitudes were compared with a behavioral measure of error detectability across two different error types in a four-choice flanker task. If an erroneous response was associated with the flankers, it was considered a flanker error, otherwise it was considered a nonflanker error. Two experiments revealed that, whereas detectability was better for nonflanker errors than for flanker errors, ERN amplitudes were larger for flanker errors than for nonflanker errors. Moreover, undetected errors led to strongly reduced ERN amplitudes relative to detected errors. These results suggest that, although error detection is necessary for an ERN to occur, the ERN amplitude is not related to error detectability but rather to error significance.