Narrative:

I called clearance delivery, got my clearance. Somehow got distraction and thought clearance had cleared me to taxi to runway 27R. About 2000 ft down the taxiway, I realized my mistake and called ground.

Google
 

Original NASA ASRS Text

Title: A KING AIR 200 ATX PLT TAXIES FROM THE TERMINAL WITHOUT CALLING GND CTLR FOR TAXI CLRNC AT OAK, CA.

Narrative: I CALLED CLRNC DELIVERY, GOT MY CLRNC. SOMEHOW GOT DISTR AND THOUGHT CLRNC HAD CLRED ME TO TAXI TO RWY 27R. ABOUT 2000 FT DOWN THE TXWY, I REALIZED MY MISTAKE AND CALLED GND.

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.