I would want to know the relation ship between islam and the number" 786" which is always attached to Islam. If praying behind an Imam and he made a mistake on the number of rakah he has prayed or missed any part to the solat but he could recollect and he cannot look back while praying how can the congregation correct him?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If the Imaam (i.e. the person leading the prayer) commits a mistake in his prayer, it is permissible for the people praying behind him to draw his attention to the mistake. The male congregation praying behind him should draw his attention by saying “Subhaana Allaah” (i.e. praise be to Allaah) whereas the female congregation should clap their hands, as the Prophet said: “When the Imaam commits a mistake, [or when something serious happens] the men can draw his attention by saying ‘Subhaana Allaah' and the women by clapping their hands.” [Al-Bukhari] If the people led in prayer say “Subhaana Allaah” and the Imaam does not understand, and there is no other alternative except by speaking to him, then one of the people led in prayer may speak out to tell him about his mistake, and the prayer does not become void by doing so.