A B737-300 IN CRUISE AT FL270 THE FLAP HANDLE WAS INADVERTENTLY MOVED TO EXTEND POS. NOTED ACFT RUMBLE AND TRANSIT LIGHTS. HANDLE MOVED TO FULL RETRACT DETENT.

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LEG #6; ACFT CHANGE IN ZZZ1 THIRD DIFFERENT ACFT. DEPARTED ZZZ1 LIGHT TO MODERATE TURB IN CLB AND DURING CRUISE. FL270 80 MI N OF ZZZ. CTR HAS GIVEN US DIRECT TO ZZZ2. I REACHED OVER TO THE FMC TO PROGRAM DIRECT TO ZZZ2. AFTER I FINISHED PUSHING THE BUTTON; MY FOREARM PULLED BACK ACROSS THE TOP OF THE FLAP HANDLE. A SECOND OR 2 LATER; I NOTICED THE AMBER FLAP TRANSIT LIGHT ILLUMINATED AND FELT A RUMBLE AND NOTICED THE FLAP NEEDLES MOVING. I PUSHED THE FLAP LEVER BACK FORWARD TOWARD THE STOP. THE FLAP NEEDLE WENT BACK TO ZERO AND THE FLAP TRANSIT LIGHT WENT OUT. MOMENTARY ALTDEV OF +100 FEET AND -50 FT. THE WHOLE EXTENSION/RETRACTION LASTED MAYBE 5 SECONDS. THE FO THEN CHKED THE FLAP LEVER AGAIN AND FOUND IT STILL NOT DOWN IN THE DETENT; SO HE MADE SURE IT WAS IN THE DETENT. REST OF THE FLT WAS UNEVENTFUL; FLAP EXTENSION WAS NORMAL. ACFT FLT NORMAL. ON TAXI IN; WE EXPERIMENTED WITH THE FLAP LEVER. IT TOOK MORE FORCE THAN NORMAL TO PUSH THE LEVER FORWARD TO THE STOP AND THEN YOU HAD TO PHYSICALLY FORCE THE HANDLE DOWN INTO THE DETENT. THE FORCE REQUIRED FOR THIS ACFT WAS MUCH DIFFERENT THAN THE OTHER 2 -700 ACFT WE HAD FLOWN THAT DAY; AND BEING NIGHT; IN TURB FROM TKOF; #6 LEG; AND A LONG DAY AT THAT; WE DIDN'T NOTICE THE DIFFERENCE. EVEN THOUGH THE FLAP GAUGE INDICATED ZERO; THE FLAP LEVER WAS NOT IN THE DETENT; SO JUST LOOKING AT THE FLAP GAUGE IS NOT FOOLPROOF EITHER. BETTER CHK THE LEVER; THEN THE GAUGE.