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. During the 1970s, numerous researchers suggested that skill learning not only results in more effective movement performance but also is manifested in terms of heightened sensitivity of the learner to detect and correct errors. Since then, several attempts have been undertaken to measure the strength of error-detection capabilities by means of subjective-error estimations (e.g., Newell, 1974; Newell & Boucher, 1974; Newell & Chew, 1974; Schmidt & White, 1972; Schmidt & Wrisberg, 1973). Using this technique, Schmidt and White (1972) found that correlations between objective and estimated error increased with practice on a ballistic-timing task, which was interpreted as the acquisition of the capability to detect errors with task experience. Various theories of motor learning (Adams, 1971; Schmidt, 1975) accounted for these findings by assuming that response-produced feedback (from the moving limbs, audition, vision, etc.) can be related to information about the success of the movement in terms of the environ