Narrative:

An aircraft small aircraft X landed on the runway. Another aircraft small aircraft Y was taxied into position to hold. I diverted my attention to handle other aircraft in the traffic pattern, then looked at the runway, didn't see small aircraft X, he was observed by a corner post of the cabin, and I mistook another small aircraft at the exit taxiway as the aircraft I didn't see on the runway. I cleared the small aircraft Y for takeoff. Another controller informed me that I had an aircraft on the runway, so I brought my attention back from sequencing another aircraft, saw the small aircraft X on the runway where it had reemerged from behind the past and aborted the takeoff clearance of the small aircraft Y, which at that time was slowly taxiing down the runway. A few moments later the small aircraft X cleared the runway and I cleared the small aircraft Y once again.

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Original NASA ASRS Text

Title: ATCT LCL CTLR CLEARED SMA FOR TKOF ON OCCUPIED RWY. GND CTLR ALERTED LCL CTLR WHO HAD SMA ABORT TKOF.

Narrative: AN ACFT SMA X LANDED ON THE RWY. ANOTHER ACFT SMA Y WAS TAXIED INTO POS TO HOLD. I DIVERTED MY ATTN TO HANDLE OTHER ACFT IN THE TFC PATTERN, THEN LOOKED AT THE RWY, DIDN'T SEE SMA X, HE WAS OBSERVED BY A CORNER POST OF THE CABIN, AND I MISTOOK ANOTHER SMA AT THE EXIT TXWY AS THE ACFT I DIDN'T SEE ON THE RWY. I CLRED THE SMA Y FOR TKOF. ANOTHER CTLR INFORMED ME THAT I HAD AN ACFT ON THE RWY, SO I BROUGHT MY ATTN BACK FROM SEQUENCING ANOTHER ACFT, SAW THE SMA X ON THE RWY WHERE IT HAD REEMERGED FROM BEHIND THE PAST AND ABORTED THE TKOF CLRNC OF THE SMA Y, WHICH AT THAT TIME WAS SLOWLY TAXIING DOWN THE RWY. A FEW MOMENTS LATER THE SMA X CLRED THE RWY AND I CLRED THE SMA Y ONCE AGAIN.

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.