It does make me cross. An owner who did the responsible thing and got her dog insured found that her policy was almost worthless because the insurers refused to pay out until they'd received a copy of the paid invoice from the vet, and the company employing her usual vet were insisting on payment up front.
For many people this wouldn't be a big deal: they'd simply pay with their credit card and pay off the card when the insurer reimbursed them. This owner was on benefits (and kudos to her for putting aside the money to pay her insurance each month in the circumstances). Hopefully she will get at least something back on the policy, as she's eligible to use our clinic and will be able to claim reimbursement of the cost of having the dog's operation done there. That doesn't make it fair that she's had nearly a week of misery thinking the dog might die if she couldn't find a way to get the operation done.
Moral: read the small print before taking out pet insurance, and ask your vet to check their practice's policy on settling bills via insurance claims.