Vedas have to be properly recited by adhering to the svaras , matras and diction.
By not conforming to the injunctions (niyamas), one would get adverse effects.
There are six types of persons whose Vedic recitation is considered defective.
Six types of adhama pAthakas with reference to Vedic recitation:
(1) One who sings without following the svara niyamas
(2) One who chants very fast
(3) One who shakes the head while reciting
(4) One who recites the Vedas using a written text (book)
(5) One who does not know the meaning of Vedas
(6) One whose voice is not audible
gItI SIghrI Sirah kampI tathA likhita pAthakah!
anarthajnah alpakantasca shadete pAthakadhamah!!