In some congregations if the Torah reader makes a mistake, the entire congregation will yell the right word. In fact, it is the job of a special gabbai (synagogue official) called a somech (supporter) — and not anyone else — to stand next to the reader (ba’al koreh) and correct his reading when necessary.
The Torah is written without vowels or te’amim, cantillation notes. The ba’al koreh must therefore memorise the pronunciation and chant of every word beforehand. Getting it exactly right is important, as a minor mistake can change the meaning of a word. For example, zachar is a male, but zecher means “in memory of”. If a reader mixes up such vowels, he must reread the word, or the entire verse if the mistake was not picked up right away.
A mistake in the tune or a vowel that does not change the meaning of the word does not require a correction (though this doesn’t stop the punters in some shuls from shouting it out anyway).