Narrative:

While holding short of runway 7L at lax, the tower asked us if we could accept runway 7R for takeoff. After conferring with the captain, I responded yes to the tower. They told us to cross runway 7L and hold short of runway 7R. I read back the clearance and the captain started across runway 7L. I then proceeded to prepare the takeoff data for the new runway with my eyes in the cockpit. When I looked up the captain had taxied onto runway 7R without clearance. I called the tower and told them we were on the runway. They sent the aircraft on final around and instructed us to remain on the runway and continue in position and hold. We were cleared for takeoff shortly thereafter. It is my opinion the captain must have misunderstood the clearance and thought we had clearance into position onto runway 7R.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR X UNAUTH RWY ENTRY CAUSED ACFT ON FINAL APCH TO GAR. RWY INCURSION.

Narrative: WHILE HOLDING SHORT OF RWY 7L AT LAX, THE TWR ASKED US IF WE COULD ACCEPT RWY 7R FOR TKOF. AFTER CONFERRING WITH THE CAPT, I RESPONDED YES TO THE TWR. THEY TOLD US TO CROSS RWY 7L AND HOLD SHORT OF RWY 7R. I READ BACK THE CLRNC AND THE CAPT STARTED ACROSS RWY 7L. I THEN PROCEEDED TO PREPARE THE TKOF DATA FOR THE NEW RWY WITH MY EYES IN THE COCKPIT. WHEN I LOOKED UP THE CAPT HAD TAXIED ONTO RWY 7R WITHOUT CLRNC. I CALLED THE TWR AND TOLD THEM WE WERE ON THE RWY. THEY SENT THE ACFT ON FINAL AROUND AND INSTRUCTED US TO REMAIN ON THE RWY AND CONTINUE IN POS AND HOLD. WE WERE CLRED FOR TKOF SHORTLY THEREAFTER. IT IS MY OPINION THE CAPT MUST HAVE MISUNDERSTOOD THE CLRNC AND THOUGHT WE HAD CLRNC INTO POS ONTO RWY 7R.

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.