AN MD80 CREW CLRING RWY AT DFW CROSSED A RWY THEY WERE TO HOLD SHORT OF.

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AFTER CLRING RWY 31R AT THE END OF TXWY P; WE WERE INSTRUCTED TO HOLD SHORT OF RWY 35C AND MONITOR THE TWR FREQ FOR THAT RWY. AS I TURNED THE CORNER ONTO TXWY Y; I WAS WATCHING A B757 TAXIING ON A PARALLEL TXWY AT R ANGLES TO OURS. ONCE ASSURED THERE WOULD BE NO CONFLICT WITH HIM AT THE INTXN (HE WAS CLRED ONTO TXWY Z) I LOOKED TO MAKE SURE OUR NOSE WAS ON THE CTRLINE OF THE TXWY Y WHILE THE FO COMPLETED THE AFTER CLRING RWY CHKLIST. WHEN I LOOKED TO THE L; I SAW THAT I HAD ALREADY STUCK MY NOSE ONTO A RWY ABOUT THE SAME TIME AS THE TWR ASKED WHO WAS THE SUPER 80 ON TXWY Y. I IDENTED MYSELF BY CALL SIGN AS I FINISHED XING THE RWY 35C AT THE VERY END OF ITS 13400 FT LENGTH. THE CTLR INSTRUCTED ME TO CALL HIM WHEN I GOT TO THE GATE; WHICH I DID QUICKLY. NO RPT WAS FILED BY ATC SINCE LAHSO INSTRUCTIONS HAD BEEN ISSUED TO THE ACFT ON RWY 35C THAT WAS COMPLETING ITS LNDG ROLL SOME DISTANCE DOWN THE RWY. I ADMITTED THE FAULT WAS MINE AS I DID NOT REALIZE I WAS APCHING THE RWY 35C SO QUICKLY. WX WAS NO FACTOR. DAYLIGHT VMC.