Aeneas Tacticus 4.8-12: Neglecting identity checks can have dire consequences.

8. Pisistratus, when general at Athens, was informed that a force coming from Megara by sea intended to attack the Athenian women by night, while they were celebrating the Thesmophoria at Eleusis. On hearing this he laid an ambush for them. 9. The force from Megara disembarked, as they thought without attracting attention, and were some way from the coast when Pisistratus burst from his ambush and overpowered them, destroying the greater number of them, and also captured their boats. 10. These he filled without delay with his own troops and, taking with him such of the women as he thought best for this purpose, put into Megara late in the evening, keeping at some distance from the city. 11. On sighting the boats, a crowd of Megarians, including all the magistrates, flocked down to meet them, seeing, as they thought, a fine cargo of female prisoners. to disembark with daggers in their hands, and to strike some of them down, but to carry off alive to the boats all the distinguished citizens they could capture. And these orders they carried out. 12. It is clear from this story that no troops should be mustered, and no expedition dispatched, without signals to ensure that the different parties are known to one another.