A300 CREW OVERSHOOTS ASSIGNED ALT DURING DSCNT TO FL240 IN ZAU AIRSPACE.

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ON VECTOR DIRECT RFD VOR AT FL330. CLRED BY ATC TO FL240. SELECTED FL240 IN ALT WINDOW. I PULLED ALT KNOB AND THEN SELECTED 'DSND NOW' ON FMS AND PROFILE MODE. ACFT BEGAN DSCNT AT 1000 FPM AND THEN CAPTURED PROFILE PATH AND DSCNT RATE INCREASED TO ABOUT 2700 FPM. COMING THROUGH FL245; ACFT PITCHED DOWN AND ACCELERATED AIRSPD AND DSCNT RATE. BOTH MYSELF AND FO NOTED THIS AND I IMMEDIATELY DISCONNECTED AUTOPLT AND PULLED NOSE UP. BY THE TIME I ARRESTED DSCNT RATE AND STARTED TO CLB; WE REACHED ABOUT FL235. I CLBED BACK TO FL240 IN ABOUT 10 SECONDS AND LEVELED THERE. RE-ENGAGED AUTOPLT AND CONTINUED NORMALLY. WE WERE BELOW FL240 AND BTWN FL235 AND FL240 FOR LESS THAN 10 SECONDS. NO COMMENT FROM ATC AND WE DID NOT DISCUSS WITH ATC. MYSELF AND FO WERE BOTH SURPRISED AND VERIFIED THAT FL240 WAS SELECTED IN ALT WINDOW. AFTER SOME DISCUSSION I BELIEVE THAT WE CAUSED THE PROB BY MAKING AN INPUT INTO FMS WHILE WE WERE STILL IN THE 'PALT' MODE AND PRIOR TO 'PALT' CAPTURED. WE HAD RECEIVED A SPD RESTR OF 280 KIAS FROM ATC AND I REQUESTED FO TO INPUT INTO FMS TACT MENU. THE INPUT WAS MADE RIGHT AFTER THE FO MADE THE FL250 FOR FL240 CALL. I SHOULD HAVE WAITED UNTIL WE FULLY CAPTURED THE PALT.