The role of acquired error-detection capabilities in skill learning was investigated by manipulating the delay of knowledge of results (KR). Compared with delayed feedback, instantaneous KR should be detrimental to the learning of error-detection capabilities because it should tend to block spontaneous subjective evaluation of response-produced feedback. Weaker error-detection capabilities should then be evident on delayed no-KR retention tests. During acquisition, one group of subjects received KR after a delay of a few seconds while another group received KR instantaneously; then both were evaluated on several retention tests. Using a timing task with two reversals in direction (Experiment 1) and a coincident-timing task (Experiment 2), we found that, relative to delayed feedback, instantaneous KR degraded learning as measured on delayed retention tests. Although the KR-delay interval has traditionally been considered of minor importance for skill learning, the present findings suggest that very short KR-delays interfere with learning, perhaps by degrading the acquisition of error-detection capabilities.