B787 technician reported they did not secure a jumper bar after completing work behind an electrical p-800 panel. The result was an air return due to warning messages and smoke in the flight deck.

Narrative

During removal of a contactor baseplate and bus bars on the rear of the p800 panel due to heat damage I removed a jumper bar from a bus bar to the back of a common motor start controller. It was removed fully from the bus bar but left loose on the mounting point to the CMSC (Common Motor Starter Controller) and not tagged or bagged; as I left it there to prevent parts from being lost. This along with a lack of information of the effected area after its removal led to the following technician missing the loose hardware; and missing the left in place loose portion of the jumper. After completion of the Maintenance visit the aircraft left for ZZZZ; and upon attempting to return to ZZZ experienced an air return due to several warnings and smoke. This was due to the aforementioned jumper not having been tightened and due to the amount of current in the system it began to fail. The top of this jumper is the failure point in question. This incident is likely due to personal complacency (assuming a mechanic would have seen it); fatigue (I transitioned from days to graveyard shifts 2 weeks ago); norms (leaving hardware loose but in place unmarked; which was due to needing to replace said attaching hardware).