A communication failure

[timelets]

Timoxeinos would write a letter to Artabazos, or Artabazos would write one to him, and they would wrap the message around an arrow beneath the point, tie it to the feathers, then shoot it to an agreed-upon place. But the plan of Timoxeinos to betray Poteidaia was detected, for on one occasion when Artabazos shot an arrow to the assigned location, he missed his aim and hit a man of Poteidaia in the shoulder instead. A crowd of people gathered around this wounded man, as tends to happen in war; they at once pulled out the arrow, and when they perceived the letter, they brought it to their generals. ”

Herodotus, Histories.

They haven't invented cyphers yet.