1. If you err or forget something while reciting Al-Fatihah, you have to correct it because the prayer for the one who does not recite Al-Fatihah is invalid. 2. If you make a mistake when reciting the Surah after Al-Fatihah or you forget anything of it, it is not prescribed for you to say Istighfar; rather you should try to correct the mistake and remember what you forgot, but if you cannot do that then you may move on to the verse after it, or leave this Surah and start reciting another, or you may bow. If you do any of these things, there is nothing wrong with that.