MIL OVERSHOT CROSSING RESTRICTION IN DESCENT RESULTING IN POTENTIAL CONFLICT WITH OPPOSITE DIRECTION TRAFFIC.

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OPERATING ON AN IFR FLT PLAN; NAS GLENVIEW TO BUCKLEY FIELD (DENVER). FLT ROUTINE UNTIL THE COPLT'S WINDSHIELD CRACKED WHEN WE WERE IN THE VICINITY OF LINCOLN; NE AT FL260. WE ALSO REALIZED OUR ALTIMETERS SHOWED A 200' SPLIT. IAW PROCS; THE COPLT DONNED HIS FLT HELMET AND PLACED HIS VISOR DOWN (HE REMAINED IN THIS ABNORMAL CONFIGN FOR THE REMAINDER OF THE FLT). WE REQUESTED AND RECEIVED AN IFR CLRNC TO RETURN TO NAS GLENVIEW. AN EMER WAS NOT DECLARED. W OF ROCKFORD; WE COMMENCED OUR DSCNT AND WERE CLRED TO 12K; EXPECT TO CROSS 10 E OF ROCKFORD AT 9K. THE 10 E OF ROCKFORD AT 9K CLRNC IS ONE WE RECEIVE ON EVERY ENTRY INTO NAS GLENVIEW FROM THE W. I DSNDED AT A SLOW RATE (1000' PER MIN) AND SLOW AIRSPD AS WE MONITORED THE WINDSHIELD. CONTINUING MY DSCNT THROUGH 12K; I NOTICED MY ALTIMETER READ 11300 WHEN CENTER GAVE US AN IMMEDIATE CLB TO 12K. THIS OCCURRED APPROX OVER ROCKFORD AS I WAS PLANNING TO BE 10 E AT 9K. I CLBED IMMEDIATELY TO 12K AND A FEW SECONDS LATER A SMALL ACFT PASSED UNDER US AT 11K. NEITHER MY COPLT NOR MY FLT ENGINEER (AN EXPERIENCED FULL TIME AT CTLR ASSIGNED TO CHICAGO CENTER) NOTICED A DISCREPANCY IN MY DSCNT; ALTHOUGH WHEN ASKED BY ATC SOON AFTER HOW WE UNDERSTOOD THE CLRNC; THE COPLT REPLIED 12K. CONTRIBUTING FACTORS: DISTR CAUSED BY CRACKED WINDSHIELD; ALTIMETER SPLIT; COPLT WEARING HEADGEAR AND VISOR; PLT AT CONTROLS INTERP CLRNC AS HE WAS ACCUSTOMED TO FLYING THIS ROUTE ON MANY OCCASIONS. IN OUR OPINION; ATC INCORRECTLY ISSUING SUCH A CLRNC; WHEN IN THE EVENT OF LOST COM; A TFC CONFLICT WOULD ARISE SINCE WE WOULD HAVE BEEN REQUIRED TO CARRY OUT 'EXPECT' PORTION.