Nedarim

R. Ishmael son of R. Jose had a vow for absolution. He went before the Rabbis, who asked him, 'Did you vow bearing this in mind?' 'Even so,' replied he. 'Or this?' 'Yes.' This was repeated several times. A fuller, seeing that he was paining the Rabbis, smote him with his basket.  Said he, 'I did not vow to be beaten by a fuller,' and so he absolved himself. R. Aha of Difti objected to Rabina: But this was an unexpected fact, as it had not occurred to him that a fuller would smite him, and we learnt: An unexpected fact may not be given as an opening?  — He replied: This is not unexpected, because scoffers  are common who vex the Rabbis. MISHNAH. R. ELIEZER B. JACOB SAID: ALSO HE  WHO WISHES TO SUBJECT HIS FRIEND TO A VOW TO EAT WITH HIM, SHOULD DECLARE: 'EVERY VOW WHICH I MAY MAKE IN THE FUTURE SHALL BE NULL'. [HIS VOWS ARE THEN INVALID,] PROVIDING THAT HE REMEMBERS THIS AT THE TIME OF THE VOW. GEMARA. But since he says, 'Every vow which I may make in the future shall be null,' he will surely not listen to him  and not come to [eat with] him? —