I have been with so many mobile operators in the past and even now with the one I have, I have this complain that they, at random, deduct data in bulk when it has not been even used. It seems to me this is some kind of profit making excercise they follow when you look at the number of users this is done to. And many users, out of either laziness and/or lack of proper tracking of their usage history don't do anything about it.