Can a federal district judge issue a warning to a witness that they are in danger of inadvertently incriminating themselves?

We agree that a district court certainly has the discretion to ensure that a witness in danger of unwittingly incriminating himself is aware of his constitutional privilege, but that exercise of this discretion must be carefully tailored to ensure that warnings do not become threats. A district judge warning a witness must be sure that she does not abuse that discretion by intimidating a witness into silence. See Webb v. Texas, 409 U.S. 95, 97-98, 93 S.Ct. 351, 353-354, 34 L.Ed.2d 330 (1972) (per curiam) (holding that trial judge committed error when he "gratuitously singled out" one witness for lengthy and intimidating warnings about perjury); Arthur, 949 F.2d at 216.