I have been a bad boy yesterday; travelling without a valid ticket on the train, which landed me a fine of €35, on top of the normal train fare.
I was quite sure I had a valid ticket, but apparently, I took an old one mistakenly.
In hindsight, it turned out that I didn't choose the correct option when I bought my tickets a few days earlier, resulting in a stack of tickets for one day, instead of several days. So basically, I paid my fare, but set to the wrong day.
I explained the whole thing in a letter to Dutch Railways; here's hoping they will cancel the fine.
Cheers, K.