If the Imam makes a Mistake in Recitation and no One corrects Him
Question:

If the Imam recites what is easy for him in the prayer and then he forgot the words of a Verse… and no one among the worshippers was able to correct him, should he make the Takbir and end the Rakah, or should he recite another Surah?

Answer:

He is free to choose, if he wishes, he may make the Takbir and end the Rakah, or, if he prefers, he may recite a Verse or Verses from another Surah according to what the pure Sunnah requires of him in the prayer in which he is reciting – if it (i.e. the mistake) is in other than Surah Al-Fatihah. As for Al-Fatihah, he must recite all of it, because its recitation is one of the pillars of the prayer. And Allaah is the Granter of success.

Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Vol. 2 Page 420

http://www.fatwaislam.com/fis/index.cfm?scn=fd&ID=1194