I heard this did happen in Canada. This was about 20 years ago, but my ex-sister-in-law who is a nurse reported that she was told by a doctor who supervised her that "if you JW's only knew how often you and your children are transfused while in the operating room without your knowledge"
Perhaps they've tightened up procedures now so that this is less likely to happen, but I agree with those who suggest doctors may be more concerned about saving a life than respecting a patient's wishes. I don't think it would have to be a huge conspiracy of silence, either. There are plenty of medical mistakes and misdeeds that go on in the hospital, and all those involved stay quiet about it for their own reasons.