Narrative:

At XA06, I issued aircraft X a 340 degree heading and 5000 ft, a vector for runway 23R at indianapolis. The pilot read back 340 degree heading and 5000 ft. Cherokee was 15 mi southeast of ind and was given a 270 degree heading and was already level at 4000 ft at the time of the vector. This was for runway 23L at indianapolis. The sector got busy and I issued the cherokee 3000 ft, which he read back. When I looked at aircraft X's tag, he was already at 4000 ft heading toward the cherokee. I issued a 020 degree heading to aircraft X and to maintain 4000 ft. I then issued an expedited descent to cherokee and gave aircraft X a turn to 050 degrees. Unfortunately, separation was lost 800 ft, 2.2 mi, approximately 10 mi southeast of indianapolis. As traffic built, more sectors should have been split off since 4 sectors were combined to my position. This might have helped in noticing aircraft X busting through his 5000 ft assigned altitude which he acknowledged, and had read back. Also, mount comfort, indiana, had an air show planned for the day so I believe, although traffic was light before the above incident occurred, it got busy fast and another sector should have been open all day, not when traffic dictated it to be open.

Related reports:

Google
 

Original NASA ASRS Text

Title: B737 ARR FAILS TO COMPLY WITH ATC ASSIGNED ALT RESULTING IN LOSS OF SEPARATION AT IND.

Narrative: AT XA06, I ISSUED ACFT X A 340 DEG HDG AND 5000 FT, A VECTOR FOR RWY 23R AT INDIANAPOLIS. THE PLT READ BACK 340 DEG HDG AND 5000 FT. CHEROKEE WAS 15 MI SE OF IND AND WAS GIVEN A 270 DEG HDG AND WAS ALREADY LEVEL AT 4000 FT AT THE TIME OF THE VECTOR. THIS WAS FOR RWY 23L AT INDIANAPOLIS. THE SECTOR GOT BUSY AND I ISSUED THE CHEROKEE 3000 FT, WHICH HE READ BACK. WHEN I LOOKED AT ACFT X'S TAG, HE WAS ALREADY AT 4000 FT HDG TOWARD THE CHEROKEE. I ISSUED A 020 DEG HDG TO ACFT X AND TO MAINTAIN 4000 FT. I THEN ISSUED AN EXPEDITED DSCNT TO CHEROKEE AND GAVE ACFT X A TURN TO 050 DEGS. UNFORTUNATELY, SEPARATION WAS LOST 800 FT, 2.2 MI, APPROX 10 MI SE OF INDIANAPOLIS. AS TFC BUILT, MORE SECTORS SHOULD HAVE BEEN SPLIT OFF SINCE 4 SECTORS WERE COMBINED TO MY POS. THIS MIGHT HAVE HELPED IN NOTICING ACFT X BUSTING THROUGH HIS 5000 FT ASSIGNED ALT WHICH HE ACKNOWLEDGED, AND HAD READ BACK. ALSO, MOUNT COMFORT, INDIANA, HAD AN AIR SHOW PLANNED FOR THE DAY SO I BELIEVE, ALTHOUGH TFC WAS LIGHT BEFORE THE ABOVE INCIDENT OCCURRED, IT GOT BUSY FAST AND ANOTHER SECTOR SHOULD HAVE BEEN OPEN ALL DAY, NOT WHEN TFC DICTATED IT TO BE OPEN.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.