Biggest mistake?. Does it mean, Krishna committed several mistakes and the question is about the biggest mistake of those several mistakes?.
Is he a common man to commit mistake?. When he was born in jail, he gave darshan to parents Devaki and Vasudeva as a child with four hands and his usual weapons. He clearly declared that he is Lord Vishnu.
He killed Pootana and other demons as an infant. He lifted Govardhan hill with a finger.
Still is there any doubt about him?.
Bhishma, the most powerful warrior on kaurava side, said “ had Krishna not been with Pandavas, I would have killed all Pandavas with a single arrow “.
Krishna is Adiguru. Guru means a person who guides disciples from darkness to light.
Krishna has not done any mistake, infact he corrects mistakes of devotees and makes their lives meaningful and fruitful.
Krishna does only right actions always.
Krishnam vandE jagadgurum.
Subhamastu