RWY INCURSION AND TRACK DEV WHEN CREW CONCEPT BREAKS DOWN.

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DURING TAXI FOR TKOF; WE HEARD TWR ISSUE WINDSHEAR ADVISORIES. THE CAPT DIRECTED A MAX PWR TKOF AND I SUGGESTED USING 'FLAPS 5 DEGS' TO CONFORM WITH THE FLT MANUAL. HE AGREED AND I PLUGGED ACARS FOR THE NEW TKOF DATA. THE BEFORE TKOF CHKLIST HAD ALREADY BEEN COMPLETED. AS WE APCHED THE HOLD SHORT (APPROX 50 YARDS AWAY) I TOLD TWR THAT WE WERE READY TO GO. I HEARD TWR CLR SOMEONE FOR TKOF. I MISSED THE CALL SIGN AND DID NOT RESPOND. THE CAPT DID NOT RESPOND EITHER BUT HE BEGAN TO CROSS THE HOLD SHORT LINE. I ASKED TWR TO CONFIRM IF WE WERE CLRED FOR TKOF. WE WERE THEN TOLD TO HOLD SHORT BY TWR. WHEN WE STOPPED WE WERE ABOUT 10-15 FT ACROSS THE LINE; BUT NOT NEARLY ON THE RWY. I TOLD TWR OUR POS AND HE SAID 'THAT'S OK; JUST HOLD SHORT.' OUR SIT DID NOT IMPROVE. I MISSED RESETTING THE FLAPS IN THE CONFUSION; BUT THE CAPT CAUGHT IT. TWR CLRED US FOR TKOF. I BELIEVED I HEARD 'FLY RWY HDG.' ONCE AIRBORNE; THE CAPT BEGAN A L TURN. HE HAD RESET HIS HDG BUG TO THE TKOF CLRNC HE HAD MISTAKENLY FOLLOWED BEFORE. I CONFIRMED OUR DEP INSTRUCTIONS WITH HIM IMMEDIATELY. AT THIS POINT WE REALIZED THAT WE WERE IN AN ERROR CHAIN. WE MADE EVERY EFFORT TO TIGHTEN AND IMPROVE OUR SITUATIONAL AWARENESS AND WE PROCEEDED WITHOUT FURTHER INCIDENT. WE WERE CORRECT TO RECONFIGURE THE ACFT FOR TKOF IN A WINDSHEAR ENVIRONMENT. WE WERE INCORRECT IN RUSHING. WE SHOULD HAVE STOPPED THE ACFT AND RERUN THE BEFORE TKOF CHKLIST. WE WERE SAVED FROM ANY INCIDENT BY GOOD ENOUGH SITUATIONAL AWARENESS TO NOTE THAT QUESTIONS NEEDED TO ASKED (WERE WE CLRED? ARE THE FLAPS CORRECTLY SET? WHAT WAS OUR ASSIGNED HDG?). FURTHER; WE EMPLOYED SOLID AIR CREW COORD/COMMAND LEADERSHIP RESOURCE MGMNT TECHNIQUES TO RECOGNIZE AND BREAK AN ERROR CHAIN.