AFP is reporting that the incident occurred in March:

A US military reconnaissance plane came under electronic attack from North Korea and had to make an emergency landing during a major military exercise in March, a political aide said Friday.

The aide said the plane suffered disturbance to its GPS system due to jamming signals from the North's southwestern cities of Haeju and Kaesong as it was taking part in the annual US-South Korea drill, Key Resolve.

The incident was disclosed in a report that Seoul's defense ministry submitted to Ahn Kyu-baek of parliament's defense committee, the aide to Ahn said.

Spokesmen for the defense ministry and US Forces Korea declined to comment.

Jamming signals -- sent at intervals of five to 10 minutes on the afternoon of March 4 -- forced the plane to make an emergency landing 45 minutes after it took off, the aide quoted the report as saying.

The signals also affected South Korean naval patrol boats and speedboats, as well as several civilian flights near Seoul's Gimpo area, according to the report.

Cell users in Seoul also reported disrupted service at the time and other GPS devices were also affected.

I wonder how often this sort of thing happens and we don't hear about it due to security concerns. We certainly don't want the North Koreans knowing if their attack was successful, and this leak appears to have been a screw up of some kind.