As the Seanchan invaded Falme, they hang the watchers of Watchers over the wave because "They watched for the wrong thing". But what is the right thing to watch? It seems to they were watching for the right thing and the army the Artur Hawkwing sent across the ocean did come back.
From WoT Wikipedia:
The Watchers Over the Waves - A group who believe that the armies Artur Hawkwing sent across the Aryth Ocean will one day return, and so keep watch from the town of Falme on Toman Head.
From The Great Hunt, Ch 29
Falme stood on a spit of land at the very tip of Toman Head, with nothing further west of it except the Aryth Ocean. High cliffs ran to the harbor mouth on both sides, and atop one of those, where every ship running into the harbor had to pass under them, stood the towers of the Watchers Over the Waves. A cage hung over the side of one of the towers, with a man sitting in it despondently, legs dangling through the bars.
“Who is that?” Domon asked.
Caban had finally given over sharpening his sword, after Domon had begun to wonder if he meant to shave with it. The Seanchan glanced up to where Domon pointed. “Oh. That is the First Watcher. Not the one who sat in the chair when we first came, of course. Every time he dies, they choose another, and we put him in the cage.”
“But why?” Domon demanded.
Caban’s grin showed too many teeth. “They watched for the wrong thing, and forgot when they should have been remembering.”