I can't speak for the correct etiquette in the Netherlands, but I can offer some general advice:
Eating at a restaurant is like entering into an unwritten or implied contract between you and the owners. The terms of this contract are that you receive food which you find both acceptable (in terms of cleanliness and presentation) and palatable (in terms of taste). In return, you give the owners the amount of money they specify on the bill.
If the restaurant fails to live up to their end of the contract, by providing food that does not meet your criteria, you are within your rights to demand a replacement. The restaurant is not obligated to do so (they can ask you to leave) but they generally won't ask you to pay for uneaten food. In most cases they won't even try, because restaurants rely on word-of-mouth, and if they get a reputation for serving bad food, it can cost them a lot more in the long run.
When sending food back, recognize your rights, and be polite but firm. There is no need to act guilty, or to make the restaurant staff feel guilty. Keep it simple, as in this advice from a restaurant owner:
When the server comes to the table, simply say it flat out: 'I ordered my eggs over-easy, but these eggs are over-hard,' or 'I ordered my steak medium-rare, but it's well done,'" he says. If you do this, "the server's response should be a genuine apology and an assurance that he or she will rectify this immediately and alert the kitchen manager of the mistake."
If your dish came with a really big blunder — an allergen made it onto your plate, or you found a foreign object, such as a hair or a nail in your food — you should take it a step further and complain, says Simpson. "Ask to speak to the manager," he advises. Then, you should "explain [what happened] and that you wanted to bring it to his or her attention.
What if the meal simply does not taste good? Hopefully, this will never happen, but if it does, again, be clear and firmly state that you do not want the food.
I can only recall one time I had to send something back, a dish of clams that tasted were so bitter and odd that I couldn't even eat them. I don't think they were spoiled, but I do think they were improperly cooked. I told the waiter:
I'm sorry, I don't mean to complain but this is not very good. Could I exchange it for something else?
The waiter apologized profusely and suggested some alternatives. The restaurant replaced my meal without any fuss, and ended up not charging me for either dish, as a way of apologizing for their error.
Side note: In places where tipping is the custom (such as in the US), even if the restaurant does not charge you for a meal, or gives you a discount, it is appropriate to calculate the tip based on what would have been the full price.