You must not go wrong either in the enunciation or intonation of a mantra. If you do, not only will you not gain the expected benefits from it, the result might well be contrary to what is intended. So the mantras must be chanted with the utmost care. There is a story told in the Taittiriya Samhita(2. 4. 12) to underline this.
Tvasta wanted to take revenge on Indra for some reason and conducted a sacrifice to beget a son who would slay Indra. When he chanted his mantra, "Indrasatrur varddhasva. . ", he went wrong in the intonation. He should have voiced "Indra" without raising or lowering the syllables in it and he should have raised the syllables "tru" and "rddha"(that is the two syllables are "udata"). Had he done so the mantra would have meant, "May Tvasta's son grow to be the slayer of Indra". He raised the "dra" in Indra, intoned "satru" as a falling svara and lowered the "rddha" in "varddhasva". So the mantra meant now: "May Indra grow to be the killer of this son (of mine)". The words of the mantra were not changed but, because of the erratic intonation, the result produced was the opposite of what was desired. The father himself thus became the cause of his son's death at the hands of Indra.
The gist of this story is contained in this verse which cautions us against erroneous intonation.
Mantrohinah svarato varnato va
Mithya prayukto na tamarthamaha
Sa vagvajro yajamanam hinasti
Yathendrasatruh svarato' paradhat
What was the weapon with which Tvasta 's son was killed? Not Indra's thunderbolt but the father's wrongly chanted mantra.
For a general background, please see here