How have courts dealt with the issue of false confessions at trial?

In Milton, several confessions were introduced at trial. One of the confessions was obtained by an undercover police officer who was placed in a cell with defendant. The court held that any error in the admission of the confession was harmless beyond a reasonable doubt. The evidence of guilt was overwhelming, and the previous, properly admitted confessions left no doubt that the jury would have reached the same verdict without hearing the challenged confession. (Milton v. Wainwright, supra 407 U.S. 371, 92 S.Ct. 2174.)