AN ACR MLG TAXIES ONTO ACTIVE RWY AT PIT; PA; WITHOUT TWR CLRNC. FLC HAS NO EXPLANATION FOR IT EXCEPT THEY WERE FATIGUED.

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WHILE TAXIING TO RWY 10C AT PIT FOR DEP; WE STAYED WITH GND CTL AS DIRECTED TO A POINT JUST PRIOR TO THE HOLD SHORT LINE. AN ACFT WAS THEN SEQUENCED AHEAD OF US AND WE WERE TOLD TO CONTINUE HOLDING FOR THE POSSIBILITY OF SOME OTHER ACFT TO GO AHEAD OF US. WE WERE FINALLY CLRED TO TAXI TO THE RWY. THE FO RESPONDED WITH A 'CLRED INTO POS AND HOLD' WHICH IS ALSO WHAT I THOUGHT I HEARD. THE FACT THAT WE WERE ALREADY SO NEAR THE HOLD SHORT LINE MADE THIS SEEM LIKE A NORMAL DIRECTION FROM TWR. HOWEVER; DURING THE DELAY; I FORGOT WE WERE STILL ON GND CTL AND NOT TWR. NOT REALIZING THE ANOMALY; I TAXIED ONTO THE RWY (FORTUNATELY IT WASN'T BEING USED FOR ARRS SO NO CONFLICT EVER AROSE). GND QUERIED US AND WE SOON REALIZED WE HAD NEVER TALKED TO TWR. WHILE I HAVEN'T LISTENED TO THE TAPES; I CAN'T EXPLAIN WHY THE FO AND I BOTH HEARD AND UNDERSTOOD A DIRECTION OF 'POS AND HOLD.' I CAN SAY THAT AS THIS WAS THE LAST LEG OF A 10 HR DAY; FATIGUE DID PLAY A ROLE IN OUR MOMENTARY LOSS OF SITUATIONAL AWARENESS AS FAR AS WHICH CTLR WE WERE WORKING WITH.