ARTCC CTLR INADVERTENTLY DESCENDED AN MLG TO AN OCCUPIED ALT IN A HOLDING PATTERN. COMBINED SECTOR OPERATION. SYSTEM ERROR.

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AT THE TIME OF THE OCCURRENCE PHOENIX HAD SHUT DOWN PHX DUE TO WX. I HAD BEEN WORKING THE D-SIDE (ASSISTANT RADAR) AND HAD MOVED OVER TO THE RADAR SIDE 3 MINS BEFORE. WE WERE HOLDING 8 ACFT; 5 AT ARLIN INTXN AND 3 AT BXK VORTAC. THE STACK AT ARLIN WAS 10000'; 11000'; 12000' AND MLG X DSNDING TO CROSS ARLIN AT 13000' WHICH WAS THE CLRNC THE PREVIOUS RADAR CTLR HAD CLRED HIM. APPROX 1 1/2 MINS BEFORE THE ERROR MLG X SAID HE WAS REDUCING TO 210 KTS. I ROGERED HIM ABOUT 30 SECS LATER AND HE SAID HE WAS GOING TO HAVE TROUBLE MAKING THE 13000' RESTRICTION. I TOLD HIM; 'DO THE BEST YOU CAN; MAINTAIN 10000'.' HE SAID '10000'; ROGER.' NEITHER I NOR MY D-SIDE CAUGHT WHAT I HAD SAID; BUT I ALREADY HAD ACFT HOLDING BTWN 10000-12000'. HE ENTERED HOLDING AND I SAW HIS ALT OF 12800'. I ASKED HIM TO SAY ALT. HE SAID HE WAS DSNDING TO 10000'. I SAID; 'NEGATIVE; MAINTAIN 13000'.' BY THAT TIME THE OTHER MLG Y WHICH WAS HOLDING AT 12000' WAS 2.5 MI TO HIS LEFT. THE C/A WENT OFF. I TURNED MLG X TO AN IMMEDIATE RIGHT TURN 290 DEG HDG AND THE OTHER ACFT TO A 190 DEG LEFT TURN. THE USE OF PROPER PHRASEOLOGY WOULD HAVE HELPED AVOID THIS SITUATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR GAVE EXPERIENCE LEVEL AND SAID WAS WORKING COMBINED SECTOR OPERATION. HAD JUST RELIEVED ANOTHER RADAR CTLR FOR A BREAK AND DID GET A GOOD BRIEFING. PROX OF TFC BY OEDP WAS 2.2 NM AND SAME ALT. SUPVR WAS STANDING BEHIND RPTR AND NONE OF THE CTLRS REALIZED THE WRONG ALT HAD BEEN GIVEN AND READ BACK. TFC ADVISORY WAS GIVEN AFTER THE FACT. C/A ACTIVATED ABOUT THE SAME TIME AS RPTR REALIZED THE ERROR OCCURRING. SUPPLEMENTAL INFO FROM ACN 96023: WE WERE TOO HIGH TO MAKE 13000' AND ALSO SLOW TO HOLDING SPD. I NOTIFIED ZAB OF THIS AND THEY STATED 'DO THE BEST YOU CAN TO 10000'.' MY REPLY WAS ROGER; MLG X CLRED TO 10000'.' THE F/O WAS FLYING AND CONTINUED TO DSND WHILE I MADE A PAX ANNOUNCEMENT. THE F/O HAD JUST ROLLED OUT OUTBND WHEN I FINISHED THE ANNOUNCEMENT AT WHICH TIME HE IMMEDIATELY TURNED AND APPLIED PWR TO THE ACFT. I ASKED WHAT WAS THE PROB AND HE REPLIED THAT THE CTLR HAD ASKED HIS ALT WHEN HE WAS AT 12200'. SHE TOLD HIM THAT HE WAS CLRED TO 13000' AND COMMANDED AN IMMEDIATE TURN TO 290 DEGS AND 13000'. I FEEL THIS WAS CAUSED BY ATC BECAUSE THE CTLR WAS WORKING TOO MANY ACFT ON HER SCREEN AND LOST TRACK OF WHO SHE WAS CTLING AND WHO SHE WAS TALKING TO. SUPPLEMENTAL INFO FROM ACN 96024: WE DEVIATED TO LAS BECAUSE OF WX. THE CAPT WAS TOLD TO CALL ZAB WHEN YOU ARRIVE. HE DID SO. THE ZAB SUPVR KNEW NOTHING OF THE INCIDENT. WE HAVE HEARD NOTHING SINCE.