When should the imam be corrected if he makes a mistake in recitation: only if the meaning changes, or when he forgets an ayah or part of it, or when he replaces a letter with another letter or makes a mistake in the diacritics (tashkeel)?

The correction of the Imam by the ma'mum (congregant) and the prompting of the Imam are permissible according to most scholars. This correction becomes obligatory if the error is in the Fatihah and deliberately making such an error would invalidate the prayer, for example, by omitting a verse or altering the meaning. It is, however, recommended if the error is in something that does not invalidate the prayer, such as a linguistic mistake (lahn) in the Fatihah that does not change its meaning, or an error in other parts of the prayer besides the Fatihah.