The Farmer's Egg
Said the Besht: "A farmer held an egg in his hand and mused: 'I shall place this egg under a hen; then I shall raise up the chick and it shall hatch other chicks; I will sell them, and purchase a cow and...' While planning thus, he squeezed the egg and it broke in his fingers.
"In the same fashion some people are satisfied with the sum of holiness and knowledge they have attained, and think constantly that they are superior to others. But they do not perceive that by doing this, they lose even the little they have attained."
Source: Kesser Shem Tov