Knowing people who have worked in fast food places, and other places where food is served. It’s generally a good idea that if you are going to complain, don’t eat anything else if they offer as you may get more “extra” ingredients than you bargained for!
Personally if the food is that bad, I would not eat it and leave it and refuse to pay and never go back. Or just leave it, pay and never go back. I’ve done both of them at different times.
I guess it depends what the problem is, I’ve been lucky and never found anything that should not be in my food. But i have been with people and they found a dark curly hair, and my sister once found a piece of glass in her curry! These places (take aways) refunded the money and offered replacement food, but would you really wanna eat from there again?… I know I could not.