What is the ruling on the prayer of someone who follows an imam who makes a mistake in saying "Sami'a Allahu liman hamidah" (Allah hears those who praise Him), and the follower sometimes says the tasmee' (Sami'a Allahu liman hamidah) and tahmeed (Rabbana walakal hamd) with the imam and sometimes differs from him?

Your prayer is valid, and your saying as a ma'mum (follower in prayer): " Sami'a Allahu liman hamidah (Allah hears those who praise Him)" does not invalidate the prayer. In fact, some scholars have stated that it is legislated for the ma'mum to also say the tasmi' (Sami'a Allahu liman hamidah), just like the imam. Furthermore, an imam's mispronunciation (lahn) in the tasmi' does not invalidate the prayer, because it is a mispronunciation that does not change the meaning and occurs outside of Al-Fatiha.