> The uncanny valley is a hypothesis in the field of aesthetics which holds that when features look and move almost, but not exactly, like natural beings, it causes a response of revulsion among some observers. The "valley" refers to the dip in a graph of the comfort level of beings as subjects move toward a healthy, natural likeness described in a function of a subject's aesthetic acceptability.

Wording by wikipedia. So apparently, there might be points on the graph where a slight increase in recognition performance actually freaks out some users.