United States v. Harden, 758 F.3d 886 (7th Cir. 2014)
A Magistrate may not accept a guilty plea. The Magistrate may conduct the plea colloquy and prepare a Report and Recommendation suggesting that the plea be accepted, but it cannot actually accept the plea.
Brown v. United States, 748 F.3d 1045 (11th Cir. 2014)
A Magistrate may not issue a final ruling on a § 2255 petition. Though a § 2255 petition is often referred to as a civil matter, it is a criminal motion for purposes of the Magistrate’s Act, 28 U.S.C. § 636.