A MD80 PILOT REPORTS RESPONDING INCORRECTLY TO A NON NORMAL AND NOT EXPECTED CLEARANCE. A TCAS TA OCCURRED BECAUSE OF A HDG DEVIATION.

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ON THE JVL ARR TO ORD; WE WERE GIVEN HOLDING INSTRUCTIONS TO HOLD IN AN ATYPICAL PATTERN AT FIX APPROX 50 NM AHEAD. THE ACFT AHEAD OF US WAS ISSUED THE SAME INSTRUCTION A FEW MINS EARLIER; BUT THE CTLR CHANGED THEIR INSTRUCTION TO A R TURN TO 360 DEGS. (BOTH THE ACFT AHEAD AND OUR FLT WERE HDG 090 DEGS; SO THIS IS A R 270 DEG TURN -- THE LONG WAY AROUND.) MINS LATER; WE WERE ISSUED THE SAME INSTRUCTION: 'XXX; TURN R; R TURN HDG 360 DEGS.' SINCE THE CAPT WAS FLYING WITH THE AUTOPLT; HE SELECTED A HDG OF 180 DEGS UNTIL THE AIRPLANE WAS HDG S; THEN SELECTED A HDG OF 270 DEGS; THEN 360 DEGS AS THE AIRPLANE WAS PASSING THROUGH A HDG OF 200 DEGS OR SO. WE DO THIS TO AVOID THE AUTOPLT FROM TURNING THE WRONG DIRECTION AS THE HDG BUG PASSES THROUGH 180 DEGS RELATIVE TO SHIP'S HDG. WHEN I SAW THE CAPT WAS USING THIS TECHNIQUE (AS GOOD PRACTICE DICTATES); I STOPPED MONITORING HIS ACTION WHEN I SAW HE HAD SELECTED HDG 360 DEGS; AS PER THE CTLR'S INSTRUCTION. I THEN WENT HEADS DOWN TO REMOVE THE HOLDING PATTERN FROM THE FMS. DURING THAT PROC; THE TCAS SYS ANNUNCIATED 'TFC; TFC.' SIMULTANEOUSLY; THE CTLR ASKED FOR OUR HDG. WHEN I LOOKED UP; WE WERE PASSING 070 DEGS. THE CTLR ISSUED AN IMMEDIATE 30 DEG L TURN TO US AND A 30 DEG L TURN TO ANOTHER FLT HEADING TOWARDS US; THUS THE TA FROM THE TCAS. THE 2 AIRPLANES PASSED WITHIN 2 MI AT CLOSEST APCH. I REALIZED THE CAPT HAD HEARD THE INSTRUCTION AS; 'MAKE A R THREE-SIXTY.' SINCE OTHER ACFT WERE BEING VECTORED DIRECT TO TEDDY INTXN (ABOUT 090 DEGS); THIS MADE SENSE TO HIM EVEN THOUGH MY READBACK WAS; 'R HDG THREE-SIX-ZERO.' I HAD MADE THE ASSUMPTION THAT THE CAPT WAS STEADY ON 360 DEGS. WORKLOAD WAS HIGH AND I HAD DECIDED THAT PRIORITY TASK WAS TO REMOVE THE HOLDING PATTERN FROM THE FMS AND SET UP THE INTERCEPT FOR THE APCH I KNEW WAS TO FOLLOW. BEING HEADS DOWN; I DIDN'T NOTICE THE AIRPLANE CONTINUING THE TURN THROUGH THE ASSIGNED HDG. I THINK THIS WAS AN HONEST MISTAKE IN INTERP/EXPECTATION IN A BUSY ENVIRONMENT UNDER NONSTANDARD CONDITIONS: 1) WE NEVER HOLD AT TEDDY IN A REVERSE PATTERN. 2) WE USUALLY EXPECT THE SAME CLRNC AS THE TFC WE'RE FOLLOWING. 3) WE ALWAYS EXPECT THAT THE PF WILL ACT ON WHAT THE PLT MONITORING READS BACK TO ATC.