CHK PLT AND NEW FO ON B737-700 EXPERIENCE MOMENTARY TRACK DEV ON COWBY RNAV SID FROM LAS. FAULT INCORRECT FMS PROGRAMMING.

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AFTER PASSING 400 FT AGL; MY FO CALLED FOR LNAV. WHEN I SELECTED IT THE FD INDICATED A LEFT TURN AND I IMMEDIATELY SELECTED HDG AND LEGS PAGE ON THE CDU. THE LEGS PAGE SHOWED THE FIRST WAYPOINT AS GUP; SO I ASKED FOR VECTORS RATHER THAN TRY TO 'FIX' THE BOX. WHEN ASKED FOR THE REASON I SAID WE ERRED AND THAT WAS IT; OTHER THAN HE SAID HE HAD TO MAKE A RPT FOR ALL RNAV ERRORS. HOW THIS HAPPENED IS A MYSTERY TO ME. I WAS CONDUCTING IOE AND THIS WAS OUR SECOND LEG TOGETHER AND HIS FIRST AS THE PF. WE HAD A MAINT ISSUE IN LAS THAT RESULTED IN A MECHANIC IN MY SEAT UNTIL SHORTLY BEFORE SCHEDULED DEP TIME. WHEN I GOT IN MY SEAT I DID MY FLOW AND CALLED FOR THE BEFORE START CHK. AFTER THE CHK WAS COMPLETE; MY FO COMPLETED THE CLRNC BRIEFING AND INCLUDED THE COWBY RNAV DEP. I THEN COMPLETED THE DEP BRIEFING AND INCLUDING A REVIEW OF THE RTE PAGES AND OUR RNAV PROCS. I DON'T REMEMBER LOOKING AT THE LEGS PAGE. WE PUSHED ON TIME AND ON INITIAL CONTACT WITH GND WE WERE TOLD WE WERE #1 FOR TKOF. AS WE APCHED THE RWY FOR DEP; I CALLED FOR THE BEFORE TKOF CHK AND WATCHED MY FO COMPLETE HIS FLOW INCLUDING THE RWY UPDATE BUTTON THAT SHOWED RWY 25R. THE ONLY POSSIBLE EXPLANATION THAT I CAN THINK OF IS MAYBE THE FLT PLAN WAS ALTERED BUT NOT EXECUTED; AND WHEN THE LOADSHEET INFO WAS EXECUTED; IT EXECUTED THE ALTERATION AS WELL. I PERSONALLY WILL CHK NOT ONLY THE RTE PAGES BEFORE DEP; BUT THE LEGS PAGE AS WELL. I WILL PAY PARTICULAR ATTENTION TO THE EXECUTE LIGHT AS WELL. I WAS AT FAULT FOR NOT HAVING THE LEGS PAGE UP PRIOR TO DEP. I AM SURE I WOULD HAVE NOTICED THE ERROR IF I HAD DONE SO.