Do you have a public transport chip card? 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 (click for English site).
Request a refund of your balance within 3 months. Step 3 for forgotten check-ins or check -outs with NS is a bit different than with other carriers. It works as follows:
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.