Professional worry warts have long wondered whether lenders' mortgage insurers would prove fair-weather friends to Australia's banks - and there's some evidence in NAB's half year profit that that may be the case.National Australia Bank said that one component of a $142 million increase in the collective provision for retail lending was a "management overlay to cover loans … where claims may not be covered by the lender's mortgage insurance providers".NAB said this mainly related to loans entered into between 2004 (the tail of Australia's housing credit bubble) and 2006.Most likely NAB's own credit standards have been found wanting under the terms of the "open book" policies of the lenders' mortgage insurance providers, under which major clients are supposed to work out whether a loan meets the insurance company's underwriting standards.It also sounds like the problem is more than just between NAB and insurers in Australia but also between Bank of New Zealand and the insurers.One of those insurers, Genworth Financial, may have maintained its AA- rating in Australia, just, but its parent company is now a mere BBB+ rated insurer and no doubt the Australian arm of the firm is looking to minimise claims by taking a tough stand.QBE Lenders Mortgage Insurance (formerly PMI) no longer has as much pressure on ratings as its major rival.NAB has a whole separate set of problems with the likelihood, or otherwise, that mortgage insurance companies won't pay up on insurance contracts in respect of various, now regretted, investments in asset-backed securities by NabCapital in the United States and Europe.