Q:

What happens when a Dutch party wins more seats than they have candidates listed for?

When looking at the list of candidates for the upcoming election for the Tweede Kamer, I can't help but feel that some parties are overly confident or overly optimistic. For instance, both the VVD and the PvdA have 80 candidates listed. And some new parties that aren't represented yet, have 30 candidates on their roster, while the polls doubt that they'll gain even one single seat.
However, unexpected wins may happen.
What happens if a party wins more seats than they have candidates listed for?
Can they bring in extra candidates? Do they have to give up the surplus seats?

A:

They lose them
If a list1 has more seats assigned to it than candidates to fill them, they lose the extra seats. Those seats are then divided among the other parties with more candidates than seats in the same way the fractional seats are assigned.
That's why parties have long lists, even if the expectations are way lower. 
There are two other reasons I can think of: Being on that list is a way for junior politicians to gain a little visibility, while some smaller parties try to draw votes by putting celebrities on the list.
1 list, not party! It's the registered, vetted and potentially combined list of candidates that counts.
References (both in dutch):
Dutch election law Article 2, paragraph 10
Wikipedia 

