A B767ER FLC EXPERIENCES A POS DEV DURING A NAV ERROR OVER THE ATLANTIC.

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FLT PLAN CALLED FOR RANDOM TRACK ACROSS N ATLANTIC VIA 4650N 4640N 4630N 4620N SILVA (4680N). THE RTE OBTAINED FROM GANDER VIA ACARS WAS IDENTICAL EXCEPT THAT POS OF 4615N WAS ADDED AND ACKNOWLEDGED. THE CAPT AND FO NEVER XCHKED BECAUSE IT WAS 46W ACROSS. AS RELIEF PLT; ALL THIS TOOK PLACE ON MY BREAK. WHEN I RETURNED AND RECEIVED MY BRIEF; THERE WERE NO CHANGES TO THE ORIGINAL CLRNC. BTWN 4615 AND 4610N SHANWICK SELCALED AND ASKED FOR OUR 4615N POS RPT. THAT'S THE FIRST WE REALIZED WE HAD MISSED A POS ON OUR RTE. ALL OUR PREVIOUS POS RPTS TO GANDER AND SHANWICK HAD OMITTED 4615N BUT THIS MISTAKE WAS NEVER CAUGHT BY CTR; GANDER OR SHANWICK. OUR POS XING 15W PUT US 7.8 NM N OF 4615N. ATTN TO DETAIL ON THE CREW AND CTR'S PART SHOULD HAVE CAUGHT THIS MISTAKE. WHEN WILL RADAR SVCS BE AVAILABLE? THE SYS AS IS; IS JUST AN ACCIDENT WAITING TO HAPPEN. SUPPLEMENTAL INFO FROM ACN 398421: DUE TO SIMILARITY OF COMPANY FILED RTE AND ACTUAL OCEANIC CLRNC; PLTS ASSUMED THE RTES WERE THE SAME AND DID NOT UPDATE FMS WITH 15W FIX. FACTOR FOR ERROR INCLUDED; BUSY WORKLOAD AS OCEANIC CLRNC WAS READ BACK; FREQ CHANGE ON PRIMARY RADIO; FATIGUE AND COMPLACENCY. ONLY GREATER DEGS OF DILIGENCE COULD HAVE PREVENTED THE ERROR. WE MESSED UP.