However, you will have title insurance anyway, which safeguards you against defects on the title.

If this is a re-sale, you should have conducted an inspection, and that inspector, should have caught the defective plumbing material and brought it to your attention to make the necessary steps to have this issue remedied, compensated or allow you to exercise the ability to walk away. 

@Rahul Sunkavalli  title insurance protects lenders against any property loss or damage that may come up because of liens, encumbrances or defects that may pop up.

The laser focus now is on the property conditon, getting your mind around potential defects, any rot, termite damage, and costs to repair or replace aspects of the home or walk away (and check the exact timelines for you to exercise this option in the contract or resolve it with the buyer)...

Someone that can review your (chain of ) title for defects?

You can manage or avoid this problem in future by structuring your deals on the acquisition side by studying title.Clouded title can run the spectrum between defective ownership by prior owners & heirs to erroneously recorded encumbrances.

So if tried to sell another property of his he wouldn't be able to without curing the defect?

.  :-) Correct....and even selling, as long as you are honest in the representing the property, and not intentionally concealing a known defect, you'll be totally fine.

So the defective signatures may not be grounds to invalidate the interest in the property. 

Couple that with varying forms of DIY paperwork, servicing and other potential loan defects and we have the grab bag of defects which warrant the discount on the loan.