If you make an error on a VAT return then you simply adjust for it on the next VAT return, providing the error does not exceed £10,000.
However, if HMRC were undertake a VAT investigation and found the error was caused due to not taking ‘reasonable care’ then they could still charge a penalty.
You can reduce the rate of any potential penalty by disclosing this to HMRC unprompted (before they find out). This can be done using the link here.
The can potentially reduce the starting rate of a penalty for a ‘careless’ error from 15% of the VAT amount to 0%.
Obviously once disclosed HMRC know about the inaccuracy. This could increase the likelihood of an investigation.