In the Patterson case, the Supreme Court held that when a suspect has been indicted but has not yet asserted his Sixth Amendment right to counsel, it is possible for police (using a standard Miranda advisement) to obtain a valid waiver of the constitutional right to counsel, after which an admissible statement could be obtained.
In the 1985 case of Oregon v. Elstad, involving a Miranda issue, the Supreme Court held that even though initial custodial questioning may have occurred without proper warnings and waiver (responses inadmissible), police could later give the warnings, obtain a waiver, repeat the interrogation, and obtain an admissible second statement. The issue in Fellers was whether officers who elicited statements from an indicted suspect without a waiver could later take a waiver and obtain an admissible second statement.
John Fellers was indicted by a federal grand jury on drug charges. Officers went to his house and arrested him. A few brief statements were elicited without any warnings or waiver. Later at the station, Fellers was Mirandized, waived, and repeated his statements. The issue was whether the second set of statements could be used at trial. The Eighth Federal Circuit Court of Appeals failed to recognize important differences between Miranda and the Sixth Amendment, and so misanalyzed the issue. Their decision was reversed by the Supreme Court.
Practical Guidelines
It's easy to see that some of these issues are too complicated for even federal appellate judges to sort out (although police officers are somehow expected not to make the same mistakes). But a few understandable rules can be taken from the Sixth Amendment cases, and should be kept in mind: