Person : I'm stuffed already, but I still have so much food left on my plate!
(Person labours on with the food)
I must not waste food! I WILL finish it! *groan*
Pratik : If you're full, leave it. There's no need to stuff yourself.
Person : HAW! How can I waste food like that!? There are so many poor people starving everyday. I'd feel so bad wasting all this food.
And then "Person" continues to finish the food, overeating uncomfortably in the process.
This is one of the most common "Stupid things that otherwise intelligent people do". 
It also gets annoying when I happen to be the person with extra food on the plate, and others give me gyaan about world hunger.
Why is it stupid?
1. Once the food is on your plate, (typically) no one else is going to get to/want to eat it.
What happens to the food on your plate has no relation whatsoever to any poor/hungry person anywhere in the world. 
- It's not like a hungry person is going to get fed if you finish the food on your plate.
- It's not like the amount of food available to the poor is going to somehow diminish because you "wasted" that extra bit of food on your plate.
2. The extra food is a bigger waste inside your body than in the bin.
Overeating for pleasure is not the smartest thing, but at least you get pleasure from it. Overeating from guilt that someone else somewhere is starving is just plain stupid!
- It's not like anyone is going to eat that food.
- It's not like you enjoy it.
- You're going to be uncomfortable for the next hour or so, and you're going to have the energy of a sleep deprived sloth.
- You're going to strain your digestive system.
- And for the majority of people (the non skinny type) you're going to eat unnecessary calories that you'll either have to work to burn off, or accumulate as fat.
Which brings us to...
Pratik's Food Wastage Principle
"If you have taken too much food, and are full... do NOT overeat just to avoid 'wasting'."
If possible/convenient, give the food to someone else to eat.
If possible/convenient, save the food to be eaten later by you.
Otherwise, just leave the food and remember to take less next time!
Calibrate, and take small portions to begin with.
Aside : Where does this come from?
So I have a theory that this tendency to "eat to not waste" gets programmed into kids when loving moms and grand moms feed the idea to the distracted child to get him to finish eating, even after his mind has wandered from food.
They (very rightly) want to teach the child to value food and not to waste it in general... which is a good rule of thumb. But, like most rules of thumb, people tend to use it without thought, and sometimes in the wrong context. 
I am NOT supporting/advocating food wastage.
Nothing should be wasted. Not food, not water, not electricity, not time. I'm still working on the time bit though... : )
 I could start a new series on my blog called "stupid things that otherwise intelligent people do". : ) (God knows there are quite a few that I do myself!)
 I'm assuming that you're not going to be able to make that left over food in your plate available to a poor hungry person. I don't see any efficient way to do this, whether the poor fellow is across the world or across the street.
 But then again, the point of a rule of thumb is to serve as a shortcut to making quick, intelligent decisions without having to start over from first principles. Hmm...
 I mention it explicitly for the benefit of those who may not get it otherwise.