I would have to say some people absolutely do. If you told a portion of the nation to keep food diaries for a week, I bet a million dollars people would leave items they've consumed off the list, simply because they forgot they ate it or don't really count it as food.
People over eat for many reasons, one that seems very common is boredom. The evening also seems to be a time when people intend to over indulge. Large portions are also to blame as is lack of knowledge. Many people don't have a clue about fat content or calories and don't want to know either.
A mars bar and a can of coke total almost 450 calories, that's a lot of calories; almost a quarter of the recommended calories for a woman per day.
Fast foods are ladened with calories. And many restaurants use oodles of fat and cream in their dishes. Even coffees and pastries can have a colossal amount of hidden calories as do pre made sandwiches.
Unless you prepare everything yourself it can be difficult to know exactly how many calories you're eating.