AN MD80 CREW; DSNDING INTO DFW; OVERSHOT ASSIGNED ALT; MISSING AN 'AT OR ABOVE' XING RESTR.

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LOCATION: KARLA FIX ON BONHAM 4 ARR INTO DFW. CAPT'S LEG. APCHING BYP; ZFW GAVE US CLRNC TO CROSS KARLA AT OR ABOVE 15000 FT AND CROSS LEMYN AT 11000 FT. FO READ BACK CLRNC AND SET 11000 FT IN ALT SELECT WINDOW. CAPT CONCURRED WITH 11000 FT SELECTION AND THE 15000 FT KARLA RESTR. DURING ALL THE VARIOUS DSCNT ACTIVITIES; CHKLISTS; PA; ETC; BOTH CAPT AND FO FORGOT ABOUT THE KARLA RESTR. PASSING 13000 FT; APCHING KARLA; CAPT REMEMBERED THE RESTR. WE WERE NOW ON APCH CTL AND TOLD THEM ABOUT OUR RESTR AND OUR ALT. APCH CTL SAID IT WAS OK AND THERE WERE NO TFC CONFLICTS. IT IS POSSIBLE THAT WE RECEIVED A FL180 DSCNT CLRNC AFTER THE KARLA AND LEMYN ASSIGNMENTS; AND WERE SUBSEQUENTLY CLRED TO 11000 FT AT LEMYN WITHOUT THE KARLA RESTR; BUT WITH ALL THE ACTIVITY I DON'T RECALL FOR SURE IF THAT WAS THE LEG ON WHICH WE RECEIVED THE FL180 CLRNC AFTER RECEIVING THE ORIGINAL STAR XING ALTS. SUPPLEMENTAL INFO FROM ACN 535786: I WAS SURPRISED WHEN THE CAPT NOTED THAT WE WERE SUPPOSED TO BE AT OR ABOVE 15000 FT AT KARLA; WE WERE PASSING THROUGH 13000 FT AT THE TIME. I IMMEDIATELY ADVISED APCH CTL OF OUR ALT AND ASKED IF THEY WANTED US TO CLB OR REMAIN AT 12800 FT. THEY SAID IT WASN'T A PROB; OR SOMETHING TO EFFECT; AND TO CONTINUE TO 11000 FT.