Every time Sisyphus, by the greatest of exertion and toil, attained the summit, the darn thing rolled back down again.
Overhanging the pool were boughs laden with luscious fruit. But each time Tantalus stretched to pluck this juicy sustenance, the boughs receded from his grasp.
Zeus chained Prometheus to a rock where each day an eagle pecked out his liver (which regenerated itself each night).
They were condemned to an afterlife of unending labor, carrying water from the Styx in leaky jars.
"That which is in locomotion must arrive at the half-way stage before it arrives at the goal."
Aristotle, Physics VI:9 (via Wikipedia)