No one is so brave that he is not disturbed by something unexpected.

[Nemo est tam fortis, quin rei novitate perturbetur.]

Julius Caesar (100-44 BC) Roman general and statesman [Gaius Julius Caesar]

The Gallic Wars [De Bello Gallico], Book 6, ch. 39 (49 BC)
    (Source)

Alt. trans.: "No one is so courageous as not to be disconcerted by the suddenness of the affair." [tr. McDevitte and Bohn]