As the car runs the ECU adapts the air/fuel mixture based on the readings from the lambda sensors. There is a limited range of adjustment and the code you have simply means it hit the limit at one point.
It's not unusual for the MIL light to come on occasionally and, as above, over filling can set it off. Also, depending on the type of high flow exhaust you might occasionally hit the limit.
The code should clear eventually but it can take 10 or more engine starts or need to be cleared in Protune. If the car's running OK it's safe to carry on driving.
