AN AIRBUS 300 DURING PUSHBACK THE FLC DISCOVERS A 25000 LB FUEL IMBAL CAUSED BY FUELER LEAVING FUELING PANEL SWITCH IN 'ON' POS.

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FUELER INADVERTENTLY LEFT REFUEL SWITCH ON AFTER COMPLETING FUELING; RESULTING IN 25000 LB FUEL IMBAL BTWN WINGS. THERE ARE NO COCKPIT INDICATIONS OF FUELING SWITCH POS EXCEPT THAT THE TRIM TANK ISOLATION VALVE INDICATES IN LINE WHEN THE WING FUELING PANEL DOOR IS OPEN. THE TRIM TANK ISOLATION VALVE FOR OUR FLT WAS CLOSED INDICATING THAT FUELING WAS COMPLETED. INDIVIDUAL FUEL TANK TOTALS AND FUEL DISTRIBUTION ARE BOTH A300 PREFLT CHKLIST ITEMS. DURING PREFLT; THE FO AND I CONFIRMED THAT THE FUEL TOTAL AND DISTRIBUTION WAS NORMAL. HOWEVER; BECAUSE OF THE IMPROPER REFUEL SWITCH POS ON THE WING FUELING PANEL; THE FUEL BEGAN XFERRING FROM ONE WING TO THE OTHER AFTER WE TURNED ON THE COCKPIT FUEL TANK PUMP WITCHES DURING NORMAL PREFLT. I WOULD LIKE TO THINK THAT THE FO OR I WOULD HAVE CAUGHT THE ABNORMAL FUEL SIT PRIOR TO TKOF; BUT I CANNOT SAY THIS WITH CERTAINTY. ONCE THE FUEL BAL IS CHKED DURING THE PREFLT CHKLIST; THE FUEL DISTRIBUTION IS NOT REFED AGAIN IN ANY PRETKOF CHKLIST. THE RED WINGTIP LOW LEVEL LIGHT I SAW DURING PUSHBACK WS THE FIRST INDICATION OF AN ABNORMAL FUEL STATE THAT I OBSERVED. FUEL TANK INDICATIONS WERE L OUTER 8000 LBS; L INNER 30000 LBS; CTR AND TRIM TANKS EMPTY; R INNER 13000 LBS; R OUTER ZERO WITH RED LOW LEVEL LIGHT ON -- WE WERE LUCKY. IF WE HAD TURNED OUR FUEL PUMPS ON LATER IN OUR PREFLT; THE FUEL COULD HAVE XFERRED ENOUGH TO CREATE A HAZARDOUS SIT WITHOUT EMPTYING THE R OUTER TANK ENOUGH TO ILLUMINATE THE LOW LEVEL LIGHT.