A B757 CREW ADMITTED THAT THEY CROSSED THE HOLD SHORT LINE FOR RWY 19R AT SNA; CAUSING TKOF CLRNC FOR ANOTHER ACR'S B757 TO BE CANCELED.

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DUE TO THE CLOSURE OF TXWY L; ALL ACFT HAD TO TAXI OUT TXWY K AND CROSS RWY 19R TO APCH RWY 19R FROM THE N SIDE FOR FULL LENGTH TKOFS. WHEN I WAS CLRED TO TAXI ACROSS RWY 19L AND HOLD SHORT OF RWY 19R; I DID NOT HEAR THE HOLD SHORT OF RWY 19R INSTRUCTION. I STARTED TO FOLLOW 2 OTHER ACFT ACROSS RWY 19R WHEN I SAW ANOTHER ACR B757 TAKING THE RWY FOR TKOF. I ATTEMPTED TO HOLD SHORT OF THE RWY; BUT MY ACFT CROSSED OVER THE HOLD SHORT LINE SLIGHTLY. CLRNC FOR TKOF WAS CANCELED AND I WAS TOLD TO CONTINUE ACROSS RWY 19R. AS I ALREADY AM AWARE; WHENEVER NORMAL OPS ARE MODIFIED; EXTREME CARE AND ATTN SHOULD BE GIVEN TO ALL ATC INSTRUCTIONS. I SHOULD HAVE BEEN ANTICIPATING CHANGES AND BEEN READY TO REACT TO THEM. SUPPLEMENTAL INFO FROM ACN 489465: I WENT HEADS DOWN TO CALL ON THE ACARS. ONCE COMPLETED; I LOOKED UP AND SAW WE WERE STILL ROLLING PAST THE HOLD SHORT FOR RWY 19R; AND IMMEDIATELY ASKED IF WE WERE CLRED TO CROSS BECAUSE I DIDN'T HEAR A CLRNC. THE CAPT STOPPED IMMEDIATELY; JUST PAST THE HOLD SHORT; BUT CLR OF THE RWY. AT THAT POINT I HEARD TWR CALL TO CANCEL A DEP CLRNC. ANOTHER ACR'S B757 WAS JUST TAKING THE RWY AT TXWY L FROM THE N SIDE ABOUT 1/2 WAY THROUGH THEIR TURN TO ALIGN. TWR THEN CLRED US TO CROSS RWY 19R AND HOLD SHORT AT OUR DEST TO DISCUSS A POSSIBLE PLTDEV AND GAVE US THE PHONE NUMBER. IN THE FUTURE I WILL NOT GO HEADS DOWN IN SUCH A CRITICAL PHASE OF GND OPS TO INSURE I AM BACKING UP THE CAPT AND MORE IMPORTANTLY; THE SAFETY OF ALL CONCERNED.