One does not associate Devi Sita with curses, but according to a story, Devi herself have cursed some entities. The whole story goes as following.
Preparation of shraddha for King Dashratha
When Rama, Sita and Lakshman went for exile, Dasharatha passed away, as he was unable to bear the pain of separation of his sons. When Rama and Lakshman got this news they started making preparation for shraddha.
Rama send his brother Lakshman to the village to get necessary things for Shraddha. But, when Lakshman didn’t return for long Rama was worried and decided to go to look for Lakshman and get the things, before leaving he make necessary security arrangements for Sita. But Rama too didn’t return for long.
Events at the time of Shraddha
Sita was worried as the time for shraddha was also passing, ceremony had to be performed before noon. Sita decided to perform the ceremony on her own with whatever ingredient that were available there.
Before starting the ceremony she bathed in the river Phalgu. She lit a earthen lamp. She then made the offering to offer it to the dead ancestors. When she had almost completed the ceremony she heard a voice, Sita you are blessed we are satisfied. she was surprised to see some hand that appeared to accept the offerings. She asked Who he was, she heard a voice that I’m your dead father-in- law, i accept your offerings and blessed her and said you have successfully completed the ceremony.
The five witnesses
Then Sita said when Rama and Lakshman will be back they will not believe her. The voice said they will have to believe you and you have five witnesses. First one is Akshaya vatam (Banyan tree), second is river Phalgu, third is a Cow, fourth is a Tulsi plant and last was a Brahmin. Saying this the hand and the voice disappeared.
Four witnesses turned back
when Rama and Lakshman were back, Sita told them the entire story about how she has successfully completed the ceremony, but both the brothers refuse to believe her. Then Sita called her witnesses,among the five only Akshaya vatam took her side and said the truth but she was surprised as river Phalgu, cow Tulsi and Brahmin denied having seen anything.
Brothers realized the mistake
Rama and Lakshman without wasting more time started the ceremony. Suddenly they heard a voice saying why have you invoked us again, Sita has performed the ceremony very religiously and satisfied us, there is no need to repeat the ceremony. Rama and Lakshmana were ashamed of not believing Sita.
Sita curses the four witnesses
Sita was very unhappy with the witnesses so she cursed them. She cursed River Phalgu that henceforth it would be flowing only underground at Gaya. She cursed Cow that henceforth its mouth will remain impure and will be no longer worshipped from front, only backside will be worshipped. She cursed Tulsi plant that there will be no more tulsi plant in Gaya. She cursed the Brahmin that he will never be satisfied. He will always be hungry and crave for more and more.
She blessed Akshaya Vatam
But, she blessed Akshaya vatam, Banyan tree to remain immortal, And all who come to gaya would perform the Pinda pradaanam at Akshaya vatam too.
But according to Shiv Purana Sita has cursed River Phalgu, henceforth it would be flowing underground, She cursed Ketaki flower that henceforth ketaki flower would not be accepted by Lord Shiva in worship. She cursed Cow that henceforth its mouth will be considered impure and lastly, she cursed Fire that it would have to consume everything indiscriminately.