CAPT OF AN MD88 OVERSHOT DSCNT ASSIGNED ALT DUE TO EITHER BUMPING THE ALT SELECT BUTTON; OR INADVERTENTLY SETTING THE WRONG ALT.

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DURING OUR DSCNT INTO DFW ON THE BYP 3 (LITTLE ROCK TRANSITION); WE WERE CLRED TO FL240. THE DSCNT WAS MADE IN VERT SPD ON THE AUTOPLT FROM FL350 TO FL240. BOTH THE COPLT AND I VERBALIZED FL240 AND POINTED TO FL240 IN THE WINDOW. GOING THROUGH FL260 WE DISCUSSED A SOUTHERN DEV DUE TO A TSTM JUST N OF THE ARR. ATC GRANTED THE L DEV AND WE MADE THE TURN IN HDG SELECT AND SLOWED OUR DSCNT RATE TO 1000 FPM. DURING THE DSCNT I WAS HAVING TROUBLE CLRING MY EARS BECAUSE OF A SINUS INFECTION. ABOUT THE SAME TIME; THE FLT ATTENDANT IN CHARGE CALLED TO ENTER THE COCKPIT TO ASK A QUESTION. AFTER I OPENED THE DOOR AND SAID HELLO; I TURNED TO CHK OUR ALT. WE WERE PASSING THROUGH FL235 WHEN I FIRST NOTICED THE PROB. I QUICKLY ROTATED THE THUMBWHEEL TO RAISE THE NOSE; BUT BY THE TIME WE STARTED CLBING WE HAD DSNDED TO FL230. RIGHT AFTER WE STARTED CLBING; ATC CALLED ABOUT OUR ALT. I DON'T KNOW IF I BUMPED THE ALT SELECT BUTTON WITH A FINGER WHILE CHANGING OUR DSCNT RATE OR IF THE COPLT BUMPED IT; BUT WE ARE BOTH SURE FL240 WAS DIALED IN. (FL220 WAS SET IN THE WINDOW.) WITH THE DISTRS OF THE TSTMS; THE FLT ATTENDANT; AND MY EARS; WE MISSED THE LEVELOFF AT FL240. I NEVER THOUGHT THE AUTO LEVELOFF WOULD NOT CAPTURE AT FL240. I WILL BE VERY SURE TO KEEP AN EYE ON THE ALT SELECT WINDOW; JUST IN CASE IT EVER GETS BUMPED AGAIN.