Also, I should amend my post regarding there being no way, IMC, that things would move toward magical interrogation based on an unverfiable utterance by a noble man. So instead of there being no way, I'll move it to highly unlikely 
Take for instance, a king has evidence of a plot to assassinate him, the commoner could not have possibly had knowledge of it, and the commoner is claiming to have overheard the noble discuss detailed plans of just such a thing. It's possible (political climate taken into account and the popularity of said noble, etc. etc.) that the King might invite said noble to the castle for a chat...
So, there are ways in which a commoner could be redeemed but he better have his facts straight. Not to mention, a king is just as likely to let the commoner suffer whatever fate at the hands of the judicial system just to buy more time in his counter plotting against the noble.

Take for instance, a king has evidence of a plot to assassinate him, the commoner could not have possibly had knowledge of it, and the commoner is claiming to have overheard the noble discuss detailed plans of just such a thing. It's possible (political climate taken into account and the popularity of said noble, etc. etc.) that the King might invite said noble to the castle for a chat...
So, there are ways in which a commoner could be redeemed but he better have his facts straight. Not to mention, a king is just as likely to let the commoner suffer whatever fate at the hands of the judicial system just to buy more time in his counter plotting against the noble.