Recent research shows that in reasoning tasks, subjects usually produce an initial intuitive answer, accompanied by a metacognitive experience, which has been called feeling of rightness. This paper is aimed at exploring the complimentary experience of feeling of error (FOE), that is, the spontaneous, subtle sensation of cognitive uneasiness arising from conflict detection during thinking. We investigate FOE in two studies with the “bat-and-ball” (B&B) reasoning task, in its standard and isomorphic control versions. Study 1 is a generation study, in which participants are asked to generate their own response. Study 2 is an evaluation study, in which participants are asked to choose between two conflicting answers (normative vs. intuitive). In each study, the FOE is measured by the FOE questionnaire. Results show that the FOE is significantly present in the standard B&B task when participants give a wrong answer, that our questionnaire can measure it, and furthermore, that it is diagnostic of genuine error.