There are no excuses for getting needlessly fat, but monitor your bodyfat or waist size on a weekly basis.
You need to eat to gain weight, though. It is usually much easier to back off on calories a tad if you notice your measurements changing in an undesirable manner than to eat at maintenance and hope for change.
For FFBs, I think the weekly measurements are key. Chances are, most FFBs have lost a lot of weight by focusing on weight and not measurements. Consequently, when they try to get bigger, the increasing scale puts images in their head. Regardless of whether you think you've gained fat or not, if the calipers or measuring tape read the same, you've stayed the same.
If you've gained weight and your arm is just a bit bigger, you've gained muscle. You can probably guess where you put on fat first, so monitor that area only. Do not mistake bulk bloat for fat gain, either. If your waist is 2 inches larger after a huge meal as compared to last Saturday morning, it's no indication of fat gain.
There is a point between undereating and gorging and that is usually just enough food to gain weight. I'm interested about the calories needed per pound of muscle, though. If muscle is 100% protein, it would about half the caloric composition of fat, though that doesn't take into account water held, etc. There is also the caloric cost of building the muscle, which is likely pretty substantial.