After the fourth Labor, to capture the Eurystheus Boar. King Eurystheus couldn't think of any other task.
The only task that the King could think of was to insult him. The king told him to clean out the stables.
Even though the task was an insult, Heracles insisted that he does it alone. He tries to trick the kind by saying you can pay me if I get this done.
You tried to trick me into paying you
That labor doesn't count since you tried to get a reward out of it.
Early in the morning as soon as they were gone, he started digging a great ditch from the beds of two rivers to the place where the stables lay.
In a few hours the rush of water had rolled all the dirt away. Heracles knocked down his own dams and blocked up his trenches, and by the time the herds came home at sunset, their whole yard was clean.
Augeas was furious because he never intended to pay Heracles.Somebody had just told him that Heracles would have cleaned the stables for nothing. So, King Augeas was going to pay him but never did. Since King Augeas said that King Eurystheus said that 5th doesn't count.