Eating less than recommended can, and can sometimes not, be a problem. However, there really is a floor - a value below which nobody should go.
And that is 1200 calories.
On less than 1200 calories worth of food, you won't get enough riboflavin, potassium, vitamin B, and etc in your diet. It's not about energy and feeling full enough on less, it's about getting all the vitamins and minerals a body needs, which doesn't change with body size and is approximately about 1200 calories worth of a balanced (nutritional, not junk including) diet for women (1500 for men).
So yes, it's a big problem to only eat 800 on a regular basis. That is starvation level.
Here are some calorie boosting tips: www.sparkpeople.com/resource/nutrition_art