A notable fuckup was a grant of immunity to Oliver North when testifying to Congress about Iran-Contra. He successfully overturned his conviction because the prosecution couldn't show that they absolutely didn't use his public testimony in preparing the ironclad case against him.

Another one is the medic who was supposed to testify against Eddie Gallagher, and was granted immunity, only to claim that he himself did the killing that Gallagher was accused of.