We missed the Norwegian VAT registration by one invoice — how bad is it?

Painful in a specific way: the threshold is NOK 50,000 rolling over 12 months, so one ordinary invoice crosses it, and VAT for the unregistered period usually cannot be re-billed to the client months later — it comes out of your own margin, plus interest. On contracting margins that is often the whole profit. If this might be you, register now and reconstruct forward; every month of waiting compounds it.