TRACON controller providing OJT noted an aircraft descended below normal assigned altitude; question...

Narrative:

I was providing training on tar-D. Traffic was in a little bit of a lull. I heard my trainee slow a DC9 behind a B757 for spacing to 250 KTS. A few minutes later; the aircraft was seen leaving 108 (mode C flashes in the automation in this area to alert controllers to descend below 110- in my opinion; this is the best patch in the entire system. The airport departures climb to 100 off the ground). I took the frequency and immediately climbed him back to 110; and the departure position that owned this airspace called on the land line shortly after we climbed him back up. Upon talking with the pilot; and he said he had been issued 70. I asked him to advise the controller if he ever gets an altitude lower than 110 on the 'long side' coming in to the airport again. He commented the he and the pilot flying both found the clearance odd; but didn't ask for clarification. We listened to the tape after we left position; and the 70 was issued clear as day. I have no excuses; I just missed it. Fatigue was not an issue. The aircraft was still 10 plus miles away from the departure path. He wasn't in proximity to any other aircraft.

Original NASA ASRS Text

Title: TRACON Controller providing OJT noted an aircraft descended below normal assigned altitude; questioned the pilot and later reviewed the recording's only to discover the aircraft was assigned the abnormal altitude.

Narrative: I was providing training on TAR-D. Traffic was in a little bit of a lull. I heard my trainee slow a DC9 behind a B757 for spacing to 250 KTS. A few minutes later; the aircraft was seen leaving 108 (Mode C flashes in the automation in this area to alert controllers to descend below 110- in my opinion; this is the best patch in the entire system. The airport departures climb to 100 off the ground). I took the frequency and immediately climbed him back to 110; and the departure position that owned this airspace called on the land line shortly after we climbed him back up. Upon talking with the pilot; and he said he had been issued 70. I asked him to advise the Controller if he ever gets an altitude lower than 110 on the 'long side' coming in to the airport again. He commented the he and the pilot flying both found the clearance odd; but didn't ask for clarification. We listened to the tape after we left position; and the 70 was issued clear as day. I have no excuses; I just missed it. Fatigue was not an issue. The aircraft was still 10 plus miles away from the departure path. He wasn't in proximity to any other aircraft.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	997902
Time
Date	201203
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Aircraft 1
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Instructor Departure Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types

TRACON controller providing OJT noted an aircraft descended below normal assigned altitude; questioned the pilot and later reviewed the recording's only to discover the aircraft was assigned the abnormal altitude.