CMI CTLR EXPERIENCED OPERROR AT 4400 FT WHILE GIVING VECTORS TO FINAL AND XFER TO ADJACENT SECTOR.

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CMI APCH CTL HAS AN E AND W SECTOR; THE BOUNDARY LINE BEING THE EXTENDED CTRLINE OF RWY 32R/14L. I WAS THE W SECTOR CTLR AT THE TIME OF THE INCIDENT. I WAS WORKING AN FA10 JET IN THE PRACTICE AREA SE OF CHAMPAIGN AT 10000 FT WHEN HE CALLED REQUESTING AN ILS RWY 32R APCH. THE E SECTOR WAS VERY BUSY WITH THE UNIVERSITY ARR RUSH AND HAD JUST ASKED ANOTHER CTLR TO SERVE AS A SPOTTER. THE FA10 HAD PREVIOUSLY BEEN POINTED OUT TO THE E SECTOR AND I THEN COORD (OFF THE RECORDED LINE) A DSCNT TO 4000 FT ON A 270 DEG HDG AND SAID I WOULD BE STANDING BY FOR A SEQUENCE FOR THE APCH. I TOLD THE FA10 TO EXPECT TO FLY ACROSS FINAL FOR THE SEQUENCE DUE TO NUMEROUS INBOUNDS TO RWY 22 (THE ADVERTISED RWYS AT THE TIME WERE RWY 14L; RWY 18 AND RWY 22). WHEN THE FA10 WAS APPROX 12 MI SE OF THE ARPT; THE E SECTOR CTLR ASKED; 'WHAT IS THE FA10 JET DOING?' I RESPONDED; 'HE'S ON A 270 DEG HDG AT 4000 FT.' AT THIS TIME; A C500 WAS DEPARTING RWY 22 ON A 140 DEG HDG CLBING TO 3000 FT. THE E SECTOR CTLR OBSERVED THE ALT OF THE FA10 DROP TO 3700 FT. THINKING THE FA10 WAS DSNDING; THE E SECTOR CTLR CLBED THE C500 TO 10000 FT AND TURNED IT TO A 110 DEG HDG. THE FA10 WENT BACK UP TO 4000 FT AND A POSSIBLE LOSS OF SEPARATION OCCURRED 6 MI SE OF THE ARPT. WHILE THIS WAS GOING ON; I WAS TALKING WITH AN INBOUND FA20 JET WHO CHECKED ON 24 MI W OF CHAMPAIGN LOOKING FOR LOWER. I OBSERVED THE FA10 W OF THE LOC AND THE C500 E OF THE LOC AND CONSIDERED THEM ON PASSING AND DIVERGING COURSES. THIS INCIDENT WAS DISCOVERED DURING A MONTHLY RANDOM AUDIT. I BELIEVE A PLTDEV TOOK PLACE. THE E SECTOR CTLR WOULD NOT HAVE CLBED THE C500 IF SHE HAD NOT OBSERVED THE FA10 DSND THROUGH HIS ASSIGNED ALT OF 4000 FT. THIS INCIDENT HAS REINFORCED THE IMPORTANCE OF DOING YOUR COORD ON RECORDED LINES TO AVOID ANY FUTURE DISPUTES. I ALSO BELIEVE THE CMI TWR CTLR SHOULD HAVE POINTED OUT THE DEPARTING C500 TO ME. A 140 DEG HDG OFF RWY 22 PUTS THE C500 AT THE BOUNDARY OF E/W AIRSPACE. IF I HAD KNOWN A C500 WAS DEPARTING RWY 22; I COULD HAVE TURNED THE FA10 TO THE SW TO AVOID ANY POSSIBLE CONFLICT.