Aratus, the Achaean strategus, having formed the project of getting Cynaetha betrayed to him, came to an agreement with those in the city who were working for him, fixing a day on which he himself was to march by night to the river that runs down from Cynaetha towards the east and remain there quietly with his forces.
Those in the city about midday, whenever they had the opportunity, were to send out quietly through the gate one of their number dressed in a mantle with orders to advance as far as a certain tomb outside the city and take up his post on it.
Meanwhile the rest of them were to attack the officers who used to keep the gate, while they were taking their midday sleep.
Upon this the Achaeans were to issue from their ambush and make for the gate at full speed.
Such being the arrangement, when the day came Aratus arrived and hid in the river-bed waiting for the signal.
But at about the fifth hour of the day the owner of some of those delicate sheep which are in the habit of grazing near the town, having some urgent private business with his shepherd, came out of the gate dressed in a mantle and went and stood on the identical tomb looking round for the shepherd.
Aratus and his troops, thinking that the signal had been given them, made a rush for the town,
but the gate was at once closed in their faces by its keepers, as their friends inside the town had as yet taken no measures, and the consequence was that not only did the coup that Aratus had planned fail, but they brought destruction on those of the citizens who were acting with him, for they were at once detected, put on their trial, and executed.
If we ask what was the cause of the disaster, the answer must be that it was the use of a single signal by the commander, who was still young and ignorant of the accuracy secured by double signals and counter signals.
On such small matters does success or failure depend in military operations.
Walbank Commentary