A LIGHT TWIN AIRCRAFT ON THE ARRIVAL TO UGN MADE AN ERRONEOUS FMC SMART TURN EARLY NEAR VEENA INTERSECTION TOWARD THE UGN RUNWAY 23 IAF. ATC GAVE THE AIRCRAFT VECTORS OUT OF ORD AIRSPACE.

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THE EVENT OCCURRED AT APPROX XA56Z I WAS THE PNF; PM. WE WERE ENRTE FROM ZZZ TO UGN DSNDING WITH MILWAUKEE APCH CTL. RTE WAS JOT-VEENA-UGN. COURSE JOT-VEENA WAS 004 DEGS. CLRNC: CROSS VEENA AT 10000 FT 250 KTS; THEN HDG 050 DEGS. ACFT; WITH AUTOPLT ENGAGED WAS DSNDING IN VERT SPD MODE WITH LATERAL NAV ENGAGED. 5 MI PRIOR TO VEENA THE ACFT LEVELED AT 10000 FT; SLOWING THROUGH 280 KTS FOR 250 KTS. MKE APCH THEN CLRED US TO 8000 FT. THE FO ENGAGED VERT SPD FOR DSCNT. WHILE DOING THIS; THE FMS 'SMART TURNED' THE ACFT TOWARD THE IAF FOR RWY 23 AT UGN. THE TURN WAS ESTIMATED TO BE 4 MI EARLY. ONCE THE FO SAW THIS HAPPENING; HE WENT TO HDG MODE WHICH WAS PRESELECTED FOR A 050 DEG HDG. HE WAS GOING TO RETURN TO A NORTHERLY HDG; BUT WE WERE SO CLOSE TO VEENA AND THE TURN WAS SO SHORT (004-050 DEGS) THAT HE LET THE ACFT ROLL OUT ON THE 050 DEG HDG. ATC THEN GAVE US A 090 DEG HDG FOLLOWED SHORTLY AFTER BY A COMMAND TO 'TURN IMMEDIATELY TO 360 DEGS.' WE COMPLIED WITH THE INSTRUCTIONS IMMEDIATELY AND WITHOUT HESITATION. THE CTLR ALSO STATED THAT SEVERAL OTHER ACFT HAVE RECENTLY DONE THE SAME THING LEADING ME TO BELIEVE THE WAYPOINT COULD BE INACCURATE IN THE FMS DATABASE. ATC ADVISED US WE WERE S OF TRACK AND IN THE ORD APCH CORRIDOR. ERROR WAS CAUSED BY FMC 'SMART TURNING' TO THE NEXT FIX EVEN THOUGH THERE WAS A RTE DISCONTINUITY AFTER VEENA INTXN. AS THE PNF THIS ALL HAPPENED VERY QUICKLY; LEAVING ME VERY LITTLE TIME TO REACT; AND THIS HAPPENED AS THE FO WAS ENGAGING A DSCNT MODE ON THE AUTOPLT. THE ERROR WAS COMPOUNDED BY THE ATC INSTRUCTION TO TURN FURTHER R TO 090 DEGS. THE WIND AT ALT WAS 330 DEGS AT 55 KTS. OUR GND TRACK PROBABLY LOOKED LIKE WE WERE PAST A 050 DEG HDG SO HE COMMANDED A TURN TO 090 DEGS WHICH ADDED TO THE TRACK ERROR INSTEAD OF CORRECTING IT. CLOSER ADHERENCE TO THE TENDENCY OF THE FMS TO 'SMART TURN' AND THE USE OF THE OVER-FLY FEATURE OF THE FMS WHICH STOPS THE EARLY TURN FEATURE.