Correcting VAT and suppletie: the €1,000 rule
Everyone makes a mistake in a VAT return now and then. A forgotten invoice, a wrong rate or a receipt in the wrong quarter. How you fix it depends on the amount. (Belastingdienst: correcting a return)
Error of at most €1,000
Did you enter at most €1,000 too much or too little? Then you may process that in your next VAT return. You add the difference to the right box (rubriek) or subtract it.
Error of more than €1,000
Is the error larger than €1,000? Then you send a correction, also called a suppletie (VAT return correction). Do it as soon as possible, but in any case within 8 weeks of discovering the error. You send the correction through Mijn Belastingdienst Zakelijk.
How far back?
You can still correct a return up to 5 years after the year it related to.
Must you report too little VAT paid?
Yes. If you paid too little VAT, you must put it right. If you paid too much, you may ask for it back.
In ZZPboek
A submitted quarter is locked, so that the figures do not change unnoticed. Found an error? Reopen the quarter and adjust the entry. The app records who reopened the quarter and when. When you reopen, the app mentions the €1,000 limit and the 8-week period.

Sources
This article is general information, not tax advice. If you are unsure about your situation, ask the Belastingdienst or an adviser.