Narrative:

Widebody transport on frequency at FL220 J70 inbound to ord. Small transport on frequency at FL250 elx-lan-ptk. I had 2 other high altitude ord inbnds who I gave standard clearance to cross 10 west of pmm at FL200. I gave small transport cross 20 west of lan at FL200. Clearance should have been 20 west at FL230. I thought I said FL230. Put FL230 hard in computer, issued traffic to both aircraft. When issued to widebody transport, I stated small transport level at FL230. Once past the widebody transport, the small transport pilot questioned his altitude. I told him to verify level FL230 (thought he blew the altitude or his mode C was inaccurate). He replied negative and was cleared to FL200. I checked the flight strip and he was correct. In my mind he was going to FL200, but I was caught in giving a repetitive clearance and said 1 thing but meant another.

Related reports:

Google
 

Original NASA ASRS Text

Title: 2 ACFT PASSED WITH LESS THAN STANDARD RADAR OR ALT SEPARATION.

Narrative: WDB ON FREQ AT FL220 J70 INBND TO ORD. SMT ON FREQ AT FL250 ELX-LAN-PTK. I HAD 2 OTHER HIGH ALT ORD INBNDS WHO I GAVE STANDARD CLRNC TO CROSS 10 W OF PMM AT FL200. I GAVE SMT CROSS 20 W OF LAN AT FL200. CLRNC SHOULD HAVE BEEN 20 W AT FL230. I THOUGHT I SAID FL230. PUT FL230 HARD IN COMPUTER, ISSUED TFC TO BOTH ACFT. WHEN ISSUED TO WDB, I STATED SMT LEVEL AT FL230. ONCE PAST THE WDB, THE SMT PLT QUESTIONED HIS ALT. I TOLD HIM TO VERIFY LEVEL FL230 (THOUGHT HE BLEW THE ALT OR HIS MODE C WAS INACCURATE). HE REPLIED NEGATIVE AND WAS CLRED TO FL200. I CHKED THE FLT STRIP AND HE WAS CORRECT. IN MY MIND HE WAS GOING TO FL200, BUT I WAS CAUGHT IN GIVING A REPETITIVE CLRNC AND SAID 1 THING BUT MEANT ANOTHER.

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.