It's a pretty big deal in that subsequent sacraments of initiation or holy orders would be invalidated. So confirmation would need to be redone as well as something like ordination to the priesthood.
That said it doesn't mean you have lived a lie or that God has failed to act in one's life. We can certainly have hope and confidence in God's love and mercy and grace in these situations. But that does not give license to simply pretend or assume a valid sacrament when there is clearly an invalid form present. So there would need to be pastoral guidance and action to remedy the situation as quickly as possible.