ZKC CTLR MISSED INCORRECT ALT READBACK ON DSCNT CLRNC AND EXPERIENCED OPERROR AT APPROX 10000 FT.

Narrative

I WAS WORKING SECTOR 54 RADAR WHEN ACFT X CHKED ON DSNDING TO FL240. I GAVE THE STANDARD XING FOR THE QBALL STAR OF 11000 FT AND 2350 KTS. WHEN ACFT X READ BACK THE CLRNC; IT WAS AROUND SHIFT CHANGE; THEREFORE; THE AREA WAS FULL OF PEOPLE COMING IN AND THE NOISE LEVEL WAS ABOVE AVERAGE DUE TO NEW CTLRS SIGNING IN TO WORK AND RELIEVING OTHER POS. ACFT X READ BACK 10000 FT AND 250 KTS AND MYSELF AND 2 OTHER CTLRS THOUGHT HE READ BACK 11000 FT AND 250 KTS. ABOUT 5 MINS LATER; I HAD AN OVERFLT ACFT AT 10000 FT XING THE ARR RTE. I ISSUED ACFT X THE TFC AND HE ACKNOWLEDGED IT; BUT WHEN I NOTICED HIM AT 10800 FT; I TOLD HIM TO MAINTAIN 11000 FT AND HE COMPLIED. SUPPLEMENTAL INFO FROM ACN 642377: I WAS OFF COM #1 GETTING ATIS; GATE ARR INFO. MY FO TOOK DSCNT CLRNC WHILE I WAS OFF-LINE. CTLR ISSUED A XING (AFTER CTR REVIEWED TAPES) OF 11000 FT/250 KIAS AT QBALL. MY FO READ BACK 10000 FT/250 KTS AT QBALL. IT WAS UNCORRECTED BY ATC. AT APPROX 11000 FT MSL; CTLR ADVISED US TO MAINTAIN 11000 FT. DSCNT WAS ARRESTED BY ABOUT 10720 FT AND WE RETURNED TO 11000 FT. ON GND; I WAS ASKED TO CALL CTR ABOUT A POSSIBLE DEV. I CALLED. THEY SAID TAPES WERE REVIEWED AND IT WAS CTLR ERROR. MY FO WAS NEW TO FLYING THE LINE. HE MADE AN HONEST MISTAKE; NOT QUESTIONING WHAT HE THOUGHT WAS A PROPERLY ISSUED CLRNC. STAR PROBABLY COULD HAVE BEEN REVIEWED MORE THOROUGHLY TO DETERMINE WHAT PROPER XING ALT SHOULD HAVE BEEN AT THAT POINT. WE COULD HAVE QUERIED ATC AS TO THE PROPER ALT. I FINISHED ON COM #2 JUST ABOUT THE TIME WE WERE APCHING 11000 FT MSL. IF I HAD FINISHED EARLIER; I MIGHT HAVE HAD TIME TO ASK ATC OR FO ABOUT OUR ASSIGNED ALT.