my aunt works as an editor for a Chinese newspaper in Vancouver
she said that if you are not happy with the waiters
you should complain only AFTER you got all your food
and if you are not happy with the food,especially when it is not clean,and you ask them to change,check carefully before you eat it

not kidding,some cooks would put some 'extra' stuffs into the food if you piss them or the waiters off,
it was on a TV show, the boss of the restaurant put a camera in the kitchen,and he saw the cooks spit on the food and put all kinds of extra stuffs into the food

my family went to a Chinese restaurant a few days ago,to celebrate the Mid-Autumn festival,the waiters were so rude,my father complained
and in the food that my brother ordered,we all tried it and we suspect the cook put some WD 40 into the food
we asked the boss to come try the food,she did not even try it in front of us,she went straight to the kitchen to talk with the cooks
my brother have taken a piece of the food home,gonna take it somewhere to examinate it soon