We all know the story behind Unisaneh Tokef tefillah that we say on the Yomim Noraim:
Rabbi Amnon, of Mainz (Mayence), Germany, in the 11th century, whom the Archbishop of Mainz, at various times, tried to convert to Christianity. On one occasion Amnon evasively asked to be given three days' time for consideration, but when he left the Archbishop's palace he immediately regretted even appearing to waver in his Jewish faith. When he failed to appear on the appointed day, the archbishop had him brought guarded into his presence. Amnon, rebuked for his failure to keep his promise, pleaded guilty, and said that his tongue should be amputated, because it had expressed a doubt as to the truth of Judaism. The archbishop, however, pronounced the sentence that Amnon's feet, which had refused to come, and his hands should be cut off. This was accordingly done.
Amnon gave orders that he be carried into the synagogue, where Rosh Hashanah was being celebrated. The reader was about to begin the Kedushah, when he was asked by Amnon to wait. The latter then recited the prayer called, from its initial words, "Unisaneh Tokef," ("Let us tell how overwhelming [is the holiness of this day]") which is a description of the Day of Judgment. No sooner had he finished the prayer than he died. Three days later he appeared to Rabbi Kalonymus (died 1096) in a dream, taught him the prayer, and asked him to spread it throughout all Jewry.
The problem with this story is that no person has the right to mutilate himself or to be asked to be mutilated. [See this sicha and the sefer לאור ההלכה where Rav Zevin has an article about Shylock in halacha [!!]]. So did it really happen? Some claim it didn't. See here.
The Lubavitcher Rebbe wrote a letter where he casts doubt on the veracity of the story:
[כרך ט' ב'תרעה]
So we will continue saying ונתנה תוקף with the knowledge that in fact the day is נורא ואיום and that our fate is decided for life or death, but without the story behind the tefilla.
Hashem is emes and Torah is emes and one must strive in all areas of his or her life to reach the emes.