Large transport given descent altitude then a second altitude, told second altitude not right.

Attributes
ACN	321000
Time
Date	199511
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 9500 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 230 flight time total : 20000 flight time type : 1600
ASRS Report	321000
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 190 flight time total : 21000 flight time type : 900
ASRS Report	321123
Events
Anomaly	altitude deviation : overshoot other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

After being assigned 11000 ft and acknowledging, we were assigned 9000 ft which I also acknowledged. Descending through 9500 ft, ATC said we had been assigned 11000 ft. We climbed back to 11000 ft and I did not question the controller further since he was extremely busy.

Original NASA ASRS Text

Title: LGT GIVEN DSCNT ALT THEN A SECOND ALT, TOLD SECOND ALT NOT RIGHT.

Narrative: AFTER BEING ASSIGNED 11000 FT AND ACKNOWLEDGING, WE WERE ASSIGNED 9000 FT WHICH I ALSO ACKNOWLEDGED. DSNDING THROUGH 9500 FT, ATC SAID WE HAD BEEN ASSIGNED 11000 FT. WE CLBED BACK TO 11000 FT AND I DID NOT QUESTION THE CTLR FURTHER SINCE HE WAS EXTREMELY BUSY.

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.