The qualitative regulation of grain size allows witnesses to increase the accuracy of their reports by adding alternatives (e.g., “the robber concealed his face with a mask, with a stocking, or with a balaclava”). However, such answers may include incompatible alternatives which may make police officers and juries distrust witnesses. In four preregistered experiments, we tested the effect of information with incompatible alternatives on witness credibility. In Experiments 1a, 1b, and 1c, we presented two short testimonies, one with incompatible alternatives and another without and credibility was lower with incompatible alternatives. In Experiment 2, we told participants that witnesses could report several alternatives and the effect was reduced. We explain the effect of incompatible alternatives on credibility from participants’ inferences that a witness producing an answer with incompatible alternatives is not confident in their memory and a failure to fully appreciate the advantages of adding alternatives.