I haven't come across any incidents of that happening within the Soviet Union itself (though keep in mind that lack of evidence isn't evidence of lack), but there was an example of it happening in Soviet dominated Bulgaria.

After the Soviet-Yugoslav split, Stalin became extremely concerned about other Communist nations in Eastern Europe taking similar actions - e.g. rejecting Soviet leadership and implementing alternative economic development plans/reforms. In order to preemptively combat "Titoist" tendencies, Stalin pushed the Eastern European countries to perform their own purges in the late 1940s / early 1950s.

The example I encountered is that of Traycho Kostov, a member of the Bulgarian politburo. During the show trial he repudiated his confession, which led to the radio broadcast of the trial to be halted and the simultaneous translation service offered to foreign journalists encountered "technical difficulties". Supposedly he repudiated his repudiation shortly before his execution.

The general trend though seems to be that the confessions had been strongly drilled into the victims of the show trials. Another incident that belied the nature of the show trials was that during a similar Titoist purge conducted in Czechoslovakia, the judge questioning Rudolf Slánský missed a question on the transcript, but Slánský responded to the missing question with the pre-assigned answer.

During the Moscow Show Trials in the Soviet Union, the coordinated nature and corroboration between the victims' confessions actually convinced some Western observers of the victims' guilt. Some prominent sympathetic European socialists also supported the results of those trials. For example the prominent Fabian Beatrice Webb wrote in the second edition of "Soviet Communism: A New Civilization?" (1937), that the show trials had helped "cut out the dead wood" within the Communist party.

Source: The Rise and Fall of Communism by Archie Brown