In the year of 1897, Swami Vivekananda stayed at the house of late Gopal Lal Seal at Cossipore. One day many pundits came to meet Swami Vivekananda in the hope of having an philosophical discussion with SwamiJi.
All the pundits were well versed in Sankrit and they were discussing the matter in fluent sankrit. So Swamiji also spoke in sanskrit with them. But at one occassion, Swamiji made a grammatical mistake in the spoken sanskrit language. He used the word Asti instead of Svasti, which made the Pundits laugh out. Swamiji quickly accpeted his mistake and the discussion went on. In the end Pundits were convinced of the Swami Vivekananda arguments and they accepted his view points and left gladly.
After the pundits left, Swami Vivekananda explained the whole discussion to the disciples and also told about the fact that they laughed at him because they had picked up a grammatical mistake in his words. Swamiji further said that as He had stayed for a long period of time away from india, so he has not spoken sanskrit for so many years. After that day disciples started speaking with Swamiji in sanskrit only.