Check the pinouts! The tab is not always ground. The 5V regulator on the UNO, MC33269D-5.0, has a tab that is 5V - the cooling pad around the part is isolated from the ground plane around it.You may need an installation kit that provides a mica insulator and insulators for the mounting screws (TO-220 package) and thermal grease to ensure good heat conduction.
Good call. I have never used one myself that isn't ground but that does not rule out the possibility. Maybe it's a euro thing.