If the police fail to read a suspect his Miranda rights and warning before questioning him following an arrest, the result is immediate (and automatic) suppression of any evidence obtained. This will have the effect of rendering whatever statements the suspect made to the police inadmissible in court. In theory, without a Miranda warning, nothing a person says in response to questioning while in custody can be used as evidence against him at trial. In practice, never mind what you see on TV, the reality can be quite different.
A violation of Miranda doctrine, in itself, is not grounds for an acquittal or a reversal of conviction. Two rules have been established in appeal procedures that are followed strictly.
1. The Harmless Error Doctrine.14 If a suspect’s involuntary confession is used as evidence at trial but there is overwhelming evidence against the suspect negating or reducing the confession’s impact on the jury, a conviction will be reversed by the superior courts.
2. The Automatic Reversal Rule.15 If an involuntary confession is admitted at trial and the constitutional violation is such that the suspect never understood his rights or was never properly Mirandized, the conviction must be reversed.
14 This is an error that does not affect a party’s substantive rights of the Case’s outcome. Black’s law
Dictionary, 7th Edition, 1999. Thomson West.
15This states that if a suspect’s involuntary confession is used as evidence at his or her trial, and it can be proved that the defendant was not properly read the Miranda Warning or did not understand the rights explicit in that warning, then the defendant’s constitutional rights have been abused and any conviction will probably be dropped. http://www.mirandawarning.org/self-incrimination.html