FLC MISPROGRAMMED THEIR FMS COMPUTER. PASSING A CONTRIVED WAYPOINT THE ACFT INSIDUOUSLY TURNED TO THE ORIGINAL WAYPOINT; REVERSING COURSE.

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BEFORE THIS INCIDENT STARTED; WE WERE FLYING FROM SEATTLE TO ANCHORAGE ON J523 TOU CLBING TO FL350. WE HAD BEEN CLRED FROM SDEA VIA J523 TOU J501 BKA J133 PANC. A FEW MI FROM TOU; CENTER CHANGED OUR FLT PLAN. WE WERE ISSUED A REVISED ROUTING WHICH CLRED US FROM OUR PRESENT POS DIRECT TO YZT THEN DIRECT TO BKA FLT PLANNED RTE TO PANC. THIS ROUTING CHANGE WAS NOT REQUESTED BY ME BUT WAS ISSUED FROM CENTER FOR THEIR OWN REASONS. I MADE THE NECESSARY CHANGES TO THE FMS AND CHKED TO BE SURE THEY WERE CORRECT. THEN I LOOKED AT THE FLT PLAN TO SEE HOW THESE CHANGES AFFECTED OUR COMPANY RPTING POINT. WE WERE EXPECTED TO RPT TO THE COMPANY YZP. BUT WE WOULD NOT FLY OVER YZP ON OUR REVISED CLRNC SO I CREATED A NEW FIX ON OUR DIRECT RTE BTWN YZT AND BKA TO USE AS A COMPANY RPTING POINT. THE NEW FIX BECAME YZP01. AFTER I CREATED THE NEW FIX; I MADE A MISTAKE. INSTEAD OF MAKING YZT THE ACTIVE WAYPOINT AS I SHOULD HAVE; I MADE THE NEW FIX YZP01 THE ACTIVE WAYPOINT. THE FIRST 3 FIXES ON THE LEGS PAGE WERE NOW YZP01; YZT; BKA. THEY SHOULD HAVE BEEN YZT; YZP01; BKA. UNFORTUNATELY; MY FO DID NOT CATCH THE MISTAKE. BECAUSE THESE 3 FIXES LINED UP SO WELL; THE MAGENTA LINE ON THE HSI LOOKED CORRECT. HOWEVER; IF I HAD CHKED THE LEGS PAGE MORE CAREFULLY; I WOULD HAVE SEEN THAT THE HDG FROM YZP01 TO YZT WAS ABOUT 120 DEGS AND I WOULD HAVE CAUGHT THE MISTAKE. THE AIRPLANE REMAINED ON COURSE AS WE PASSED OVER YZT BECAUSE THE DIRECT RTE TO YZP01 TOOK US OVER YZT. AT LEAST IT WAS CLOSE ENOUGH THAT ONE NOTICED; INCLUDING VANCOUVER CENTER. AS WE APCHED YZP01; WE WERE FLYING ABOVE A CLOUD DECK AND OCCASIONALLY WE FLEW THROUGH THIN LAYERS OF CIRUS CLOUDS. WE HAD NO GND CONTACT AND VERY LITTLE HORIZON LINE. BY NOW; IT WAS TIME TO MAKE THE COMPANY RPT. WE USUALLY MAKE THESE RPTS WITH ACARS; BUT ACARS DOES NOT WORK IN THIS AREA SO WE HAD TO MAKE A VOICE RPT. WHILE WE WERE BUSY GETTING THE INFO FOR THE COMPANY RPT AND LOOKING FOR THE CORRECT FREQ TO USE IN THIS AREA; THE AIRPLANE REACHED YZP01. BECAUSE WE WERE OCCUPIED; NEITHER ONE OF US NOTICED THAT THE AIRPLANE WAS TURNING TO THE R; GOING BACK TO YZT WHICH WAS NOW THE ACTIVE WAYPOINT. VANCOUVER CENTER CALLED AND ASKED WHERE WE WERE GOING. AS I LOOKED BACK TO THE INSTS; I WAS HORRIFIED TO SEE THE HDG OF THE AIRPLANE COMING THROUGH 110 DEGS WHICH I KNEW AS NOT CORRECT. MY FO TOLD CENTER WE WERE HAVING A NAV PROBLEM AND ASKED FOR A HDG TO BKA. CENTER GAVE US THE HDG OF 300 DEGS WHICH I TURNED THE AIRPLANE TO IMMEDIATELY. AFTER CONFIRMING THAT THE NEW HDG WAS CORRECT; WE FLEW TO PANC WITHOUT FURTHER INCIDENT.