DUE TO DISTRS AND FATIGUE; FLT CREW OF B737-900 FORGET TO SET FLAPS TO TKOF SETTING. REJECT TKOF AT LOW SPD WHEN CONFIGN WARNING SOUNDS.

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ON TKOF ROLL WE RECEIVED AN AURAL TKOF CONFIGN WARNING HORN AT A VERY LOW SPD; THE CAPT CALLED 'ABORT I HAVE IT.' WE TOLD TWR AND TAXIED OFF THE RWY. ASSESSING THE SIT THE FLAPS HAD NOT BEEN POSITIONED CORRECTLY. WE CORRECTED THE PROB. WE THEN MADE AN UNEVENTFUL TKOF. NO OTHER CORRECTIVE ACTIONS REQUIRED. SUPPLEMENTAL INFO FROM ACN 620123: AT THE VERY MOMENT I WOULD HAVE CALLED FOR FLAPS 5 DEGS; OPS CALLED WITH AN AMENDMENT TO OUR TKOF WT. THE FO IMMEDIATELY REPLIED TO OPS AND WAS BUSY COPYING THE REVISION ONTO THE DISPATCH RELEASE WHILE I CLRED L AND R AND BEGAN A VERY SLOW TAXI OUT. I NEGLECTED TO MAKE THE FLAPS 5 DEG CALL BECAUSE OF THE OBVIOUS INTERRUPTION TO THE FO. I FEEL 2 THINGS WOULD HAVE PREVENTED THIS OCCURRENCE. THE FIRST IS FOR ALL AMENDMENTS TO DISPATCH RELEASES TO BE XMITTED VIA ACARS. ACARS CAN BE ANSWERED AT OUR LEISURE AND DOES NOT INTERRUPT FLOWS AND CHKLISTS AS MUCH AS A CALL FROM OPS. THE SECOND IS TO NOT LET COMPLACENCY CREEP INTO THE MIX NO MATTER HOW TIRED YOU ARE.