If, when wiring devices with a particular circuit, you click on a device that has already been assigned a different circuit, you are presented with a dialog box stating "create a multiple branch circuit? Yes/No?
This happens to me a lot when I copy an existing fixture to a new location but forget to clear the circuit assignment.
It would be awesome if the dialog box had a third choice such as Yes / No / Re-Assign, so you could easily change the device to the new circuit. Right now, you have to cancel out the wiring operation, open the properties for the device, clear the circuit assignment and then return to wiring.