Narrative:

Air carrier X at FL240 requested higher altitude due to light to moderate turbulence. I told him to expect higher shortly and inadvertently issued 'maintain FL250,' and issued traffic 10 mi west of his position (an air carrier Y jet) descending to FL250. My intent was to have him maintain FL240, but said 'FL250' probably, because I looked at the air carrier Y data block. I thought I was saying FL240. The traffic was average, I had other things to accomplish, but it was not overly complex or busy. I believe if I was a bit more conscientious and slower in my clearance, I would have been cognizant of what I said and what he read back.

Related reports:

Google
 

Original NASA ASRS Text

Title: ZLA CTLR EXPERIENCED OPERROR AT FL250 WHEN ISSUING INCORRECT ALT ASSIGNMENT TO ENRTE ACFT AND MISSING READBACK.

Narrative: ACR X AT FL240 REQUESTED HIGHER ALT DUE TO LIGHT TO MODERATE TURB. I TOLD HIM TO EXPECT HIGHER SHORTLY AND INADVERTENTLY ISSUED 'MAINTAIN FL250,' AND ISSUED TFC 10 MI W OF HIS POS (AN ACR Y JET) DSNDING TO FL250. MY INTENT WAS TO HAVE HIM MAINTAIN FL240, BUT SAID 'FL250' PROBABLY, BECAUSE I LOOKED AT THE ACR Y DATA BLOCK. I THOUGHT I WAS SAYING FL240. THE TFC WAS AVERAGE, I HAD OTHER THINGS TO ACCOMPLISH, BUT IT WAS NOT OVERLY COMPLEX OR BUSY. I BELIEVE IF I WAS A BIT MORE CONSCIENTIOUS AND SLOWER IN MY CLRNC, I WOULD HAVE BEEN COGNIZANT OF WHAT I SAID AND WHAT HE READ BACK.

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.