Mistakes when checking in/out is the most likely reason.

Thanks for your answer! Is there any way to get the money back?

Contact the customer service. You can find the various options to contact them on the NS site. 

uitcheckgemist.nl - usually works fastest and easiest.

BUT there are now an awful lot of complaints from people who tell that their travel-history is not available-if this a wide problem then most likely it may also concern you and then it simply wont work. Noone will know when its Ok again.

It should go without saying that the normal fare will be deducted.

its quite an achievement to even be able to pass gates at both sides to miss a check.

Uitcheckgemist.nl doesn't work for forgotten check outs at NS as far as I know. If you've got a Mijn NS account it should be possible to correct your forgotten check out there.