CAPT OF AN MLG ACR ACFT TAXIED ONTO THE WRONG RWY FOR TKOF RESULTING IN ANOTHER LNDG ACFT TO BE SENT AROUND BY THE TWR. UNAUTH RWY ENTRY AT MIA.

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UPON CALLING MIA GND CTL FOR TAXI; WE WERE CLRED TO TAXI DOWN THE TXWY THAT PARALLELS RWY 12 (I DON'T HAVE MY MANUAL WITH ME; SO I CAN'T IDENT TXWY BY NAME); AND TO EXPECT RWY 12 FOR DEP. AS WE GOT CLOSER TO THE RWY END AND STILL HAD NO WT AND BAL INFO FROM COMPANY OPS VIA ACARS; THE CAPT TOLD ME TO CONTACT COMPANY ON THE RADIO TO TRY AND SPD THEM UP WITH OUR WT AND BAL NUMBERS. I WAS; AS A RESULT; OFF OF THE GND CTL FREQ DURING THIS TIME. WHEN I FINISHED TALKING WITH COMPANY OPS; I TOLD THE CAPT I WAS BACK WITH HIM. HE TOLD ME THAT GND CTL SAID OUR SEQUENCE WAS BEHIND THE SAAB UP AHEAD OF US. AS WE GOT NEAR THE AREA WHERE BOTH RWY ENDS ARE CLOSEST TO EACH OTHER; THE CAPT STARTED SWINGING THE ACFT OVER TO THE R TO LINE UP BEHIND THE SAAB WHO WAS; IT LOOKED LIKE; GOING TO BE USING RWY 9L. AT THIS TIME; I QUESTIONED THE CAPT AND SAID I THOUGHT GND CTL HAD TOLD US TO EXPECT RWY 12 FOR TKOF DURING OUR INITIAL CALL UP. HE REPLIED THAT WHILE I WAS OFF THE AIR; HE TOLD US RWY 9L BEHIND THE SAAB. SINCE I WAS OFF THE AIR AND HAD NO REASON TO DOUBT HIM; I ACCEPTED HIS REPLY WITHOUT ANY FURTHER THOUGHT. AFTER THE SAAB TOOK OFF; THE TWR CTLR TOLD US TO POS AND HOLD. I ACKNOWLEDGED POS AND HOLD AS THE CAPT PROCEEDED TO LINE UP ON RWY 9L. AS WE WERE TAKING THE RWY; ALL 3 OF US COMMENTED ON THE AIRPLANE ON FINAL APCH (WE HAD A JUMPSEAT RIDER WITH US); AND HOW IT WAS HARD TO TELL WHICH RWY HE WAS LINED UP ON; 12 OR 9L. (OF COURSE; WE ALL ASSUMED RWY 12 SINCE WE WERE 'CLRED' ONTO 9L.) JUST AS WE WERE LINING UP ON THE RWY; THE TWR TOLD US TO MAKE A 180 DEG TURN AND GET OFF THE RWY; AND AT THE SAME TIME TOLD THE LNDG ACFT TO GAR. LATER THE CAPT SAID HE WASN'T SURE IF GND CTL SAID; 'YOUR SEQUENCE IS BEHIND THE SAAB;' OR 'YOUR SEQUENCE IS BEHIND THE SAAB FOR RWY 9L.' WAS HIS INSTRUCTIONS TO FOLLOW THE SAAB JUST FOR TAXIING OR FOR TAXIING AND TKOF? PERSONALLY; I SHOULD HAVE MADE A QUICK CHK WITH GND TO CONFIRM OUR DEP RWY SINCE I HAD 2 PIECES OF CONFLICTING INFO. AND LUCKY FOR US; IT WAS A CAVU DAY. I WILL DOUBLE-CHK FROM NOW ON ANY CONFLICT ON SUCH IMPORTANT INFO.