I made also last year the same name mistake: I wrote only my first name and because in my passport there are all my 3 first names I paid 80 euros for the name change.
And it was totally my own fault because when one is buying a ticket the is a sentence: "Type all names and last names in full as stated in the ID that will be presented for boarding."
Alla namees mean all names. It is my (!) responsibility to check that I write the same name with all details as it is in my passport. If I write only my first name it is not the same name as in the passport. That´s it.