Apparently alot of the foods/meals replace fat with sugar/salt. The other thing was diet drinks. The aspartame fools the brain into thinking it's going to get a load of energy, but because it isn't sugar you tend to eat more to get the energy. They had 2 rugby teams who could drink as much coke as they wanted before a buffet.(i team on ordinary, 1 on diet) The ones on diet coke ate an extra 142 cals each.
Obviously if you only do the occasional meal/ drink it won't harm, but I had colleagues who lived on ww meals and diet coke.