Not just officers, anyone in special operations, and or in any of the other "critical" fields.
I have a friend who was involuntarily held 2 1/2 years past his ETS date and sent to iraq to boot. I also have several friends who have been held for less time, or have been recalled after having been out for like 1-3 years.
Believe me, it is something I am acutely aware of every day. I am just waiting for a "congratulations on your reuping letter to arrive, or worse, I know of at least one case where a firend came out of his house in the morning and their was a car waiting for him. (He speaks like 5 languages including everything you might need in the middle east)