We use this proverb about those who violate treaties, "The Locrians and the pact," and the origin of this is that, as both authors and other people agree, on the occasion of the invasion of the Heracleidae the Locrians had promised the Peloponnesians to raise war signals in case it happened that the Heracleidae tried to cross by Rhion and not to pass the Isthmus, so that due warning might be given and measures taken to prevent their invasion.
The Locrians, however, did not do this, but on the contrary raise friendly signals when the Heracleidae arrived, so that they made the crossing in safety, and the Peloponnesians, thus betrayed by the Locrians and neglecting to take any precautions, before they were aware of it had permitted their foes to enter their country.
Walbank Commentary