Narrative:

I was the radar controller working inbound traffic to pbi at the time the incident occurred. Air carrier X and cpr Y, the aircraft involved, were #3 and #4 respectively being vectored for descent and spacing. Air carrier X was being stepped down over cpr Y. At 1 point I asked cpr Y his altitude because I had lost his mode C readout. He reported he was level as assigned at 10000'. I then inadvertently cleared air carrier X to descend and maintain 10000', fully intending to clear him only to 11000'. I could not enter the altitude into the computer because pbi approach control had track control which I cannot override. Therefore, the data block showed last assigned altitude of 12000'. I was not aware of the error until I saw air carrier Y descend through 11000'. I immediately told him to maintain 11000'. Closest proximity was 2.5 NM and 500' vertical.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR X HAD LESS THAN STANDARD SEPARATION FROM CPR Y. SYSTEM ERROR.

Narrative: I WAS THE RADAR CTLR WORKING INBND TFC TO PBI AT THE TIME THE INCIDENT OCCURRED. ACR X AND CPR Y, THE ACFT INVOLVED, WERE #3 AND #4 RESPECTIVELY BEING VECTORED FOR DSNT AND SPACING. ACR X WAS BEING STEPPED DOWN OVER CPR Y. AT 1 POINT I ASKED CPR Y HIS ALT BECAUSE I HAD LOST HIS MODE C READOUT. HE RPTED HE WAS LEVEL AS ASSIGNED AT 10000'. I THEN INADVERTENTLY CLRED ACR X TO DSND AND MAINTAIN 10000', FULLY INTENDING TO CLR HIM ONLY TO 11000'. I COULD NOT ENTER THE ALT INTO THE COMPUTER BECAUSE PBI APCH CTL HAD TRACK CTL WHICH I CANNOT OVERRIDE. THEREFORE, THE DATA BLOCK SHOWED LAST ASSIGNED ALT OF 12000'. I WAS NOT AWARE OF THE ERROR UNTIL I SAW ACR Y DSND THROUGH 11000'. I IMMEDIATELY TOLD HIM TO MAINTAIN 11000'. CLOSEST PROX WAS 2.5 NM AND 500' VERT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.