The acquisition of number conservation is a critical step in children's numerical and mathematical development. Classic developmental studies have established that children's number conservation is often biased by misleading intuitions. However, the precise nature of these conservation errors is not clear. A key question is whether conservation errors result from a failure to inhibit the misleading intuitions or from a failure to detect that these intuitions are erroneous in the first place. The present paper focuses on this fundamental conservation error or conflict sensitivity issue. We review behavioral and neuroimaging work that suggests that non-conserving preschoolers detect that their response is questionable and are less ignorant about conservation than their well-documented errors might have previously suggested.