ZFW HDOF CTLR WITNESSED OPERROR AT FL270 DURING HOLDING OPS WHEN DATA BLOCK INFO WAS NOT UNDATED AND SEPARATION WAS LOST.

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TSTMS HAD TEMPORARILY CLOSED OFF OUR CORNER-POST TO ZZZ2 TERMINAL ARRS AND 6-8 ACFT WERE IN HOLDING WAITING FOR THE WX TO CLR SO WE COULD RESUME ARRS. WE WERE TOLD TO BRING IN THE ZZZ1 ARRS AND THEN WITHIN 2 MINS WE WERE TOLD TO PUT THE ZZZ1 JETS BACK INTO HOLDING. THEN WERE TOLD TO BRING IN THE ZZZ2 JETS AND AGAIN WERE TOLD TO PUT THEM BACK INTO HOLDING. AT ABOUT THIS TIME I WAS ADDED TO THE SECTOR AS A HDOF CTLR. THE RADAR CTLR DECIDED TO LOWER ACR X IN THE HOLDING STACK; SO WE COULD HOLD MORE ACFT. THE ACR X WAS ASSIGNED A HDG TO FLY FOR HIS DSCNT AND CLRED FOR FL250; BUT THE DATA BLOCK WAS NOT UPDATED CORRECTLY; IT SHOWED FL280. ACR Y WAS IN THE SAME HOLDING STACK AT FL270. THE ACR X WAS ASSIGNED A SECOND HDG FOR HIS DSCNT AND LATER CLRED TO THE HOLDING FIX. IN READING BACK THE HOLDING CLRNC THE PLT RESTATED 'DSNDING TO FL250.' WHEN THE ACR X TURNED TO ENTER HOLDING HE DSNDED THROUGH FL270 AND WE LOST SEPARATION WITH ACR Y. WHEN THE ACR X WAS ASKED HIS ALT HE REPLIED 'FL265 DSNDING TO FL250.' THERE WAS NO ADVANTAGE TO SENDING THE ACFT BACK TO FL280 LIKE WE THOUGHT HE WAS ASSIGNED; SO THE CTLR TOLD HIM TO DSND TO FL250. THE ACR X PLT REPLIED WITH 'WE ARE HAVING TO COME BACK AROUND HERE TO GEENI' (THE HOLDING FIX). THE PLT MAY HAVE REALIZED THERE WAS A PROB WITH HIS CLRNC; BUT HE DIDN'T QUESTION THE CLRNC. LESSONS: UPDATE THE DATA BLOCKS AT ALL TIMES. IN THIS CASE THE ACR X WAS ASSIGNED 2 HDGS FOR HIS DSCNT THAT WERE NOT IN THE DATA BLOCK. THESE REMINDERS ARE THERE; SO THE CTLR HAS A VISUAL REMINDER OF WHAT EACH ACFT IS DOING. THE DSCNT TO FL250 WAS ALSO NOT CORRECTLY ENTERED INTO THE DATA BLOCK. ACFT ARE ALLOWED WIDE LATITUDE TO SET UP IN A HOLDING PATTERN WHEN IT IS NOT PART OF AN ARR THAT THEY ARE ALREADY FLYING. ANY PLT WHO HAS REASON TO QUESTION HIS CLRNC SHOULD NEVER HESITATE TO CALL THE CTLR AND VERIFY. THE SMALL ADDITION TO THE WORKLOAD IS MINOR COMPARED TO THE WORK TO FIX A MISTAKE LATER. PLTS ARE TOLD 'FIRST: FLY THE PLANE; ALL ELSE IS SECONDARY.' THEY ARE REMINDED THAT DISTRS CANNOT ALLOW THEM TO LOSE TRACK OF THE #1 PRIORITY. FOR CTLRS THE SAME THING IS 'FIRST: SEPARATE THE PLANES; ALL ELSE IS SECONDARY.' WE NEED TO REMEMBER AT ALL TIMES THAT WE CANNOT BE DISTR BY ANYTHING LIKE TMU OR HELPING ANOTHER CTLR UNTIL SEPARATION IS TAKEN CARE OF.