Narrative:

I was cleared by ground to runway 25L via taxiway east; hold short of runway 25R. Upon holding short; ground told me to switch to tower frequency for departure from current position. I contacted tower; unaware he wanted me to depart on runway 25R. I asked to cross runway 25R via taxiway east to runway 25L and he corrected me; saying I wanted a departure. I said yes I wanted a departure; and he told me to hold short. Then he issued a clearance for taxiway east at runway 25R. I questioned; saying 'you want me to take off 'on' taxiway east and make left turns for landing on runway 25L?' he affirmed my question; so I read back the same thing again; cleared the txwys and runways for traffic and departed on taxiway east. No other aircraft were within a distance to the point of compromising safety. After the tower told me; 'that's not what I wanted you to do;' I followed instructions and taxied back to the ramp. In my fault; I was aware that operations don't usually; if ever; authorize a taxi takeoff clearance; but the airspace was busy and I knew they had special operations being conducted; so upon second confirmation I departed. The tower did not phrase my clearance correctly and did not catch the error in my readbacks. I believe both of us were at fault in the incident.

Google
 

Original NASA ASRS Text

Title: A C172 PLT COMPLETED A TKOF ON TXWY E AFTER MISUNDERSTANDING ATC'S CLRNC FOR TKOF ON RWY 25R FROM TXWY E.

Narrative: I WAS CLRED BY GND TO RWY 25L VIA TXWY E; HOLD SHORT OF RWY 25R. UPON HOLDING SHORT; GND TOLD ME TO SWITCH TO TWR FREQ FOR DEP FROM CURRENT POS. I CONTACTED TWR; UNAWARE HE WANTED ME TO DEPART ON RWY 25R. I ASKED TO CROSS RWY 25R VIA TXWY E TO RWY 25L AND HE CORRECTED ME; SAYING I WANTED A DEP. I SAID YES I WANTED A DEP; AND HE TOLD ME TO HOLD SHORT. THEN HE ISSUED A CLRNC FOR TXWY E AT RWY 25R. I QUESTIONED; SAYING 'YOU WANT ME TO TAKE OFF 'ON' TXWY E AND MAKE L TURNS FOR LNDG ON RWY 25L?' HE AFFIRMED MY QUESTION; SO I READ BACK THE SAME THING AGAIN; CLRED THE TXWYS AND RWYS FOR TFC AND DEPARTED ON TXWY E. NO OTHER ACFT WERE WITHIN A DISTANCE TO THE POINT OF COMPROMISING SAFETY. AFTER THE TWR TOLD ME; 'THAT'S NOT WHAT I WANTED YOU TO DO;' I FOLLOWED INSTRUCTIONS AND TAXIED BACK TO THE RAMP. IN MY FAULT; I WAS AWARE THAT OPS DON'T USUALLY; IF EVER; AUTHORIZE A TAXI TKOF CLRNC; BUT THE AIRSPACE WAS BUSY AND I KNEW THEY HAD SPECIAL OPS BEING CONDUCTED; SO UPON SECOND CONFIRMATION I DEPARTED. THE TWR DID NOT PHRASE MY CLRNC CORRECTLY AND DID NOT CATCH THE ERROR IN MY READBACKS. I BELIEVE BOTH OF US WERE AT FAULT IN THE INCIDENT.

Data retrieved from NASA's ASRS site as of January 2009 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.