There are some slideshows on this topic going around the internet at the moment. They list foods that are generally regarded as healthy, such as dried fruit, protein bars, and low fat dessert alternatives such as yoghurt. They also suggest margarine is less healthy than butter because it contains a lot of trans fats.

'Dangerous' seems rather a strong word, though; if you look at these foods, they just tend to be higher in calories or carbs than you might expect. A better word is maybe 'deceptive' because these are all foods that people buy in the belief that such foods will do them no harm at all.

It seems to me that the lesson from this is: be very sceptical about 'low fat' claims and specially manufactured 'health' products. Basically, the healthiest food is usually what you cook yourself, using fresh ingredients and not much fat, sugar etc. You can find some great advice here.