1. Check the Note and Mortgage: They must match as to debt, date, parties. In order for there to be a mortgage, there HAS to be an underlying debt. If there is an error in the Note, there may not be an underlying obligation to support the Mortgage.
2. Look at the Assignment of the Note and Mortgage: Did the party bringing the foreclosure own the paper at the time the foreclosure was filed? If not, there was no "standing" or right of the Plaintiff to bring suit. If the Mortgage was assigned but not the Note, you can argue that the Mortgage is defective - again there has to be an underlying debt to support a mortgage. If the party holding the Mortgage does not have the right to the underlying debt, they likely don't have a valid mortgage.
Some things to look for: Was the assignment drafted BEFORE the foreclosure or AFTER? If AFTER, Plaintiff likely has no right to bring suit. Was the law firm that brought the foreclosure the drafter of the Assignment? If so, it is likely they have a conflict of interest under their Disciplinary Rules. They can't both represent the grantor and the recipient of the Assignment without full disclosure of the conflict of interest.
3. Look at the affidavits. Is it signed by a party who has authority with the Lender? Does it state what that authority is? Does it comply with the Rules for the jurisdiction in which the foreclosure is brought?
4. Look at whether the legal description is correct. The lender has to foreclose on the correct property.
5. Failure to follow TILA, RESPA and Truth in Lending statutes might delay entry of judgment and might provide set offs of amounts owed, but doesn't necessarily prevent the right of the Lender to foreclose.