LTT CREW HAD TXWY INCURSION AT IAH; AFTER A READBACK HEARBACK PROB.

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AFTER LNDG RWY 8; WE EXITED THE RWY ON HIGH SPD TXWY NK. WE WERE ADVISED TO CONTACT GND CTL ON 121.70. GND CTL WAS VERY BUSY. WE WERE INSTRUCTED 'ACR XYZ; L TURN TXWY NA; TXWY NN HIGH SPD.' WE PROCEEDED TXWY NA TO TXWY NN AND STARTED A R TURN TXWY NN. GND WAS READ BACK THOSE INSTRUCTIONS. ONCE WE ENTERED TXWY NN; GND ANNOUNCED QUICKLY HE HAD ADVISED US TO HOLD SHORT TXWY NN HIGH SPD. NEITHER PLT HEARD THE HOLD SHORT PART OF THE INSTRUCTION. WE STOPPED QUICKLY. THIS ALLOWED ONCOMING TFC TO CONTINUE ACROSS THE EAST BRIDGE WHICH WAS TILL OVER 150 YARDS AWAY. HOWEVER; ACFT LNDG RWY 8 PLANNED ON EXITING AT TXWY NN; WHICH CONTINUED TO THE END OF RWY 8. THE PROB WAS DISCOVERED IMMEDIATELY UPON ENTERING THE INTXN OF TXWY NK AND TXWY NN. CONTRIBUTING FACTORS WAS MOST LIKELY FREQ CONGESTION; WHICH PREVENTED US FROM HEARING THE HOLD SHORT PORTION OF THE TAXI INSTRUCTION. THIS COULD HAVE BEEN PREVENTED BY MONITORING TAXI READBACK INSTRUCTIONS AND NOTICING WE DID NOT REPEAT BACK WHAT WAS ACTUALLY INSTRUCTED ACCORDING TO THE GND CTLR. ALL HOLD SHORT INSTRUCTIONS ARE TO BE READ BACK. HOWEVER; IF THE INSTRUCTION IS PARTIALLY BLOCKED AND THE HOLD SHORT PORTION IS NOT READ BACK TO THE CTLR; THE CTLR IS THE ONLY REDUNDANCY [SAFETY VALVE] IN PREVENTING THIS FROM OCCURRING.