Do you have an OV-chipkaart? If so, you already know that you need to check in and out for every trip. Nevertheless, it's possible to miss checking out. This is why there is now uitcheckgemist.nl (in Dutch).
Remember to request your refund within three months. It works as follows:
Step 1: register
Go to uitcheckgemist.nl and sign in using your OV-chipkaart number. If you have a personal OV-chipkaart, also enter your birthdate.
Step 2: enter info
The website lists all rides without a check out. Note: It may take up to 14 days before you see your missed check out refund. If after this period your trip still does not show up, you can try again after a few days.
Want to request a refund for a trip? Enter the boarding location and the destination. The balance will be displayed immediately.
Step 3: collect
Your requested balance refund will be readied. You can reload it on your OV-chipkaart at an add value machine and then use it to travel.
The money that remains over from all claims is used in projects and measures to improve public transportation and in this sense flows indirectly but always back to the travellers where it belongs.