Narrative:

Aircraft X inbound to teterboro on metropolitan 4 arrival. Pilot given a clearance to cross 15 mi southwest of ard (yardley) at 12000 ft. Controller emphasized 12000 ft since this is not the usual crossing altitude. Pilot read back 'cross 15 mi southwest of ard at 11000 ft.' this was missed by me, the controller, air carrier X even asked about the rides on the way down. I again told him that it would be a smooth ride 'on your way down to 12000 ft.' no response. Aircraft X and air carrier Y were both at 11000 ft approximately 1 mi away from each other, near as I could tell. There are severe target and data block jumps in this area due to a years-old radar sort box problem. Had I had even a remote idea as to where aircraft X was when the incident occurred (or before it occurred), I would have rechked his altitude assignment and corrected a potentially catastrophic situation from developing. Matters being such as they are, I wasn't worried about the 2 aircraft because the computer told me that even if they were at the same altitude, there was enough time to do something before separation was lost.

Related reports:

Google
 

Original NASA ASRS Text

Title: RPTED LOSS OF SEPARATION WHEN ACFT X CONFLICTS WITH ACR Y AT 11000 FT AFTER RPTR MISSED ACFT X'S INCORRECT ALT XING READBACK OF 11000 FT INSTEAD OF 12000 FT.

Narrative: ACFT X INBOUND TO TETERBORO ON METRO 4 ARR. PLT GIVEN A CLRNC TO CROSS 15 MI SW OF ARD (YARDLEY) AT 12000 FT. CTLR EMPHASIZED 12000 FT SINCE THIS IS NOT THE USUAL XING ALT. PLT READ BACK 'CROSS 15 MI SW OF ARD AT 11000 FT.' THIS WAS MISSED BY ME, THE CTLR, ACR X EVEN ASKED ABOUT THE RIDES ON THE WAY DOWN. I AGAIN TOLD HIM THAT IT WOULD BE A SMOOTH RIDE 'ON YOUR WAY DOWN TO 12000 FT.' NO RESPONSE. ACFT X AND ACR Y WERE BOTH AT 11000 FT APPROX 1 MI AWAY FROM EACH OTHER, NEAR AS I COULD TELL. THERE ARE SEVERE TARGET AND DATA BLOCK JUMPS IN THIS AREA DUE TO A YEARS-OLD RADAR SORT BOX PROB. HAD I HAD EVEN A REMOTE IDEA AS TO WHERE ACFT X WAS WHEN THE INCIDENT OCCURRED (OR BEFORE IT OCCURRED), I WOULD HAVE RECHKED HIS ALT ASSIGNMENT AND CORRECTED A POTENTIALLY CATASTROPHIC SIT FROM DEVELOPING. MATTERS BEING SUCH AS THEY ARE, I WASN'T WORRIED ABOUT THE 2 ACFT BECAUSE THE COMPUTER TOLD ME THAT EVEN IF THEY WERE AT THE SAME ALT, THERE WAS ENOUGH TIME TO DO SOMETHING BEFORE SEPARATION WAS LOST.

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.