Gautama’s curse on the brahmanas who wished to leave, creating a deception as an excuse to do so. From Varaha Purana.
In ancient times, the sage Gautama performed austerities in Dandakaranya forest. Lord Brahma became pleased and gave him some divine seeds capable of producing eternal crops. Gautama then went to Saptasring mountain and made his hermitage there. He sowed the seeds and was very pleased to see rice grains sprouting up within moments. Gautama lived there happily with his disciples. In due course of time, the whole country was plagued by a great famine — everywhere except at the hermitage of Gautama.
One day, a few sages arrived at Gautama’s hermitage, and he treated them with all respect. These sages enjoyed the hospitality of Gautama, staying at his hermitage and engaging in long discussions with him, until eventually the famine was over. Then they decided to go back to their respective native places. When they sought Gautama’s permission, he requested them to stay for a few more days.
The sages then decided to use deceitful means for taking their leave of Gautama’s association. They created a superficial cow with the help of their illusionary powers and left it near the hermitage of Gautama.
Gautama was very pleased to see that cow. He eulogised it by sprinkling water on it. Hardly had he finished his act than the cow died. All the deceitful sages then returned to the scene, and cursed Gautama for killing an innocent cow. They told him that they could never live along with a person who had killed a cow, and on this excuse, they took their leave, just as they had meant to do all along.
Gautama then strictly observed the prescribed atonement for such an inauspicious act as killing a cow. Afterwards he discovered that the cow had simply been an illusion created by the brahmanas as part of their deception. He angrily cursed them to lose their Vedic knowledge. In this way, knowledge turned to ignorance.