LGT TAXIES INTO POS AND HOLD WHEN INSTRUCTED TO HOLD SHORT. FORCES GAR.

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I WAS THE FO ON A B-757; FLT FROM LAX TO MEX. WE TAXIED FROM TERMINAL 5 TO RWY 25R AND WERE INSTRUCTED TO HOLD AT TXWY 8; A DESIGNATED ATC HOLDING POINT. THE TAXI PHASE HAD BEEN WITH A SINGLE ENG IN ACCORDANCE WITH CURRENT COMPANY PROCS. ALSO DURING THE TAXI PHASE WE DISCOVERED THAT OUR FMC HAD LOST SOME OF OUR FLT PLAN DATA FOR UNKNOWN REASONS; THIS DATA THEN HAD TO BE RELOADED. AS WE APCHED THE TAXI HOLD POINT RELOADING OF THE FMC WAS COMPLETE BUT NOT VERIFIED; AT THIS SAME TIME THE CAPT CALLED FOR START OF THE SECOND ENG; WHICH I INITIATED. ALSO AT THIS TIME THE CAPT BEGAN A VERY LENGTHY 'WELCOME ABOARD' ANNOUNCEMENT TO THE PAX WHICH WAS HIS NORMAL PROC. AS HE COMPLETED HIS ANNOUNCEMENT TWR CTL CALLED AND ISSUED US TAXI INSTRUCTIONS. OVER THE TOP OF THIS ATC RADIO XMISSION THE CAPT VERY LOUDLY SAID TO ME; 'I GOT IT' THEN ACKNOWLEDGED ATC. I CONTINUED WITH MY CHKLIST AND FINISHED VERIFYING THE RELOADED FLT PLAN DATA AS WE TAXIED TOWARDS THE END OF THE RWY. THE ONLY PART OF THE ATC RADIO XMISSION THAT I CLRLY HEARD WAS THE INITIAL IDENT OF OUR FLT NUMBER; THE REST WAS BLOCKED BY THE VERBAL INTERRUPTION FROM THE CAPT. I COMPLETED THE FLT PLAN DATA VERIFICATION AS WE TURNED FROM THE TXWY TO THE FLT PLAN DATA VERIFICATION AS WE TURNED FROM THE TXWY TO THE RWY AND STARTED THE 'BEFORE TKOF' CHKLIST AS ASKED FOR BY THE CAPT. WHEN WE WERE IN POS ON THE RWY AND HOLDING I HEARD A COMMUTER ACFT CALL A GAR DUE TO AN ACFT ON THE RWY. IT WAS AT THIS TIME THAT I REALIZED THAT WE HAD BEEN INSTRUCTED TO TAXI FULL LENGTH AND HOLD NOT POS AND HOLD. SOME ADDITIONAL EVENTS AND FACTORS THAT HAD OCCURRED EARLIER IN THE DAY; THAT I FEEL WERE CONTRIBUTING FACTORS LEADING TO THIS INCIDENT ARE AS FOLLOWS: OUR DAY HAD STARTED WITH A LATE DEP FROM SFO TO SLC DUE TO THE LATE ARR OF OUR INBOUND ACFT. ALL SEATS WERE FULL AND MOST PAX WERE MAKING FLT CONNECTIONS IN SLC. THE ENRTE WX WAS NOT GOOD; SOME TURB; NUMEROUS TSTMS; AND CONTINUOUS COORD WITH ATC FOR DEVS FROM OUR RTE OF FLT FOR WX AVOIDANCE. SHORT TURNAROUND TIME IN SLC WHICH INVOLVED AN ACFT CHANGE LOCATED A TERMINAL DIFFERENT FROM THE ONE WHICH WE ARRIVED. (LONG WALK.) FLT FROM SLC TO LAX MUCH THE SAME AS THE FIRST. LATE DEP; FULL AIRPLANE; NUMEROUS FLT CONNECTIONS; AND CONTINUOUS WX AVOIDANCE. ON ARR LAX THE JETWAY MALFUNCTIONED AND IT TOOK 10-15 MINS FOR GND CREWS TO POS THE JETWAY SO PAX COULD DEPLANE. MEANWHILE WE HAD TO REASSURE THE PAX THAT EVEN THOUGH WE WERE LATE AND TEMPORARILY BEING HELD PRISONERS ON THE AIRPLANE; THEY WOULD STILL MAKE THEIR CONNECTIONS. ANOTHER SHORT TURNAROUND TIME FOR THE FLT TO MEX. INCLUDING ANOTHER ACFT CHANGE. OUR DAY WAS ONLY HELD OVER AT THIS POINT BUT ALREADY FILLED WITH HIGH WORKLOADS AND MANY STRESSFUL SITS. IN ADDITION THERE WAS A LACKING IN CREW COORD; MGMNT OF FLT DECK DUTIES; AND OPEN COMS. SUPPLEMENTAL INFO FROM ACN 280480: BOTH CREW MEMBERS STRESS FATIGUED FROM 2 HRS OF TSTM EVASION AND ATC PROBS PRIOR 2 LEGS.