A B737-300 CREW; ON TKOF; EXPERIENCED THE COCKPIT DOOR COMING OPEN.

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BEGAN TKOF ROLL AT APPROX XA55. AT APPROX 100 KIAS; COCKPIT DOOR OPENED. THE DOOR WAS CLOSED BY THE FIRST FLT ATTENDANT APPROX 2 MINS LATER AFTER AIRBORNE. THERE WERE NO SECURITY INCIDENTS OR KNOWN ADVERSE PAX REACTIONS TO INCIDENT. POSSIBLE CONTRIBUTING FACTORS: 1) CREW BECAME DISTR JUST PRIOR TO PUSHBACK DUE TO A PAX DEPLANING ISSUE AND POSSIBLE IMPACT ON AN ON-TIME DEP. 2) PRIOR TO DSCNT; CREW HAD FLOWN 5 SUCCESSIVE LEGS OVER 2 DAYS ON ACFT WITH THE NEW FORTRESS DOOR AND NO SECURITY BAR. 3) CREW DUTY DAY HAD BEGUN IN MFR WITH INCIDENT AT DEN OCCURRING APPROX 9 HRS LATER. FATIGUE MAY HAVE BEEN AN ISSUE. BOTTOM LINE: THE FACT THAT WE HAD AN UNMODIFIED DOOR WITH A SECURITY BAR FLEW UNDER OUR RADAR SCREEN DEN. CRM FACTORS MAY HAVE BEEN DISTR; CREW FATIGUE AND THE FACT THAT PREVIOUS LEGS ON THIS IDENT WERE FLOWN EXCLUSIVELY WITH THE NEW DOOR AND NO SECURITY BAR. BOTH CAPT AND FO AGREED THAT WITH 2 DIFFERENT DOOR SYS NOW ON THE COMPANY B737 FLEET; THAT WE BOTH NEEDED TO DEVELOP A HABIT PATTERN DURING PREDEP THAT INCLUDED A VISUAL CHK OF THE COCKPIT DOOR PRIOR TO PUSHBACK.