Without having more insight about this particular case than what I read from the media (and that can be quite confusing), a 12 degree list sounds reasonable - and serious enough.
When steaming at normal sea speed on a ship like the extacy, or any modern cruise ship, one should be very careful when changing course. The rudder angle should never exceed 5 degrees and one should keep a sharp eye on the rate of turn indicator (how fast the ship turns).
Passenger ships are sensitive to these kind of manouvers because of the relatively large distance between the centre of gravity and the metacentre (a point intercepted by a vertical line through the centre of bouyancy). When a force is affecting this, for example when a rudder angle increases rapidly, this force causes a list on the ship. Other ship types like tankers, are not that sensitve to these kind of forces as they are lower and wider.
Reading about this makes me remember a similar incident with the Norwegian Sky back in 2001, enroute to Seattle after an Alaska cruise. What happened then, was that the rudders all of a sudden went all the way to one side, causing a list of about 12-18 degrees before they managed to regain control over the ship. 12 degrees list sounds small, but it isn't. Standing on the bridge, one can easily feel when a ship is listing more than 0,5 degrees to either side.