A B737 COCKPIT DOOR WAS INADVERTENTLY LEFT UNLOCKED DURING PREFLT AND SUBSEQUENTLY OPENED DURING THE TKOF ROLL.

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UPON RETURNING FROM OPS WITH THE PAPERWORK FOR THE FLT I FOUND THE FIRST FLT ATTENDANT AND THE FO STANDING OUTSIDE THE LOCKED FLT DECK DOOR. AFTER ENTERING THE CODE; WE WERE ABLE TO REENTER THE FLT DECK. AS THE FO AND I ENTERED THE FLT DECK WE BOTH CONFIRMED THAT THE PWR SWITCH FOR THE DOOR WAS IN THE DOWN POS. AT SOME POINT THEREAFTER; SOMEONE LIFTED THE SWITCH TO THE OFF POS. NEITHER OF US NOTICED THIS AS WE WERE BUSY GETTING THE FLT DECK READY AND DEALING WITH ANOTHER CREW MEAL WHICH CATERING HAD FAILED TO DELIVER. PUSHBACK; START; AND TAXI WENT FINE; BUT AS WE STARTED OUR TKOF ROLL; I HEARD WHAT I THOUGHT WAS A MEAL CART SLIDE BACK AGAINST ITS STOP. A FEW MOMENTS LATER I HEARD WHAT SOUNDED LIKE THE TOILET SEAT FALLING DOWN. THIS TURNED OUT TO BE THE FLT DECK DOOR SWINGING OPEN THEN BEING SLAMMED SHUT. SINCE THE DOOR WAS UNPWRED; IT SWUNG OPEN AGAIN. THE FLT ATTENDANT REPEATEDLY SLAMMED THE DOOR DURING INITIAL CLB. AFTER AT LEAST HER 4TH ATTEMPT; I PICKED UP THE PA AND REQUESTED THAT SHE STAY IN HER SEAT AND LEAVE THE DOOR ALONE; WHICH SHE DID. I MADE THIS CHOICE TO AVOID FURTHER DISTR AND TO PROTECT THE FLT ATTENDANT IN THE ROUGH CONDITIONS I EXPECTED TO ENCOUNTER ON DEP. (THE ARR HAD BEEN VERY ROUGH.) AS SOON AS WE WERE A FEW THOUSAND FT ABOVE THE GND; I CALLED THE FLT ATTENDANT ON THE INTERPHONE AND REQUESTED SHE GET UP; FLIP THE SWITCH DOWN; AND SHUT THE DOOR. THERE WAS NO FURTHER INCIDENT. IN RETROSPECT; I THINK WE MISSED A LOT OF OPPORTUNITIES TO CORRECT THE SIT BEFORE IT BECAME A PROB: THE PERSON WHO PLACED THE SWITCH IN THE OFF POS SHOULD HAVE INFORMED; OR BETTER YET ASKED; THE PLTS. I SHOULD HAVE NOTICED THE AMBER LIGHT ON THE DOOR CTL PANEL WHILE SETTING THE TRIM IN THE PREFLT. FINALLY; THE FO SHOULD HAVE CAREFULLY CHKED THAT THE SWITCH WAS DOWN ON HIS PRE-TAXI CHK FLOW.