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There are numerous ways that such a scenario might or might not impact a case.  But the short answer is that such an event may have an impact on the informant's credibility if he testifies at trial or it might impact the prosecutor's decision as to whether to go forward with the case before trial.  On the other hand, depending on what other evidence there may be in the case, it may have no impact at all.  Any person who has been charged and is facing a trial on charges that resulted from this informant's actions or testimony shopuld have an attorney and that attorney needs to be aware of these facts to move forward with the defense.