The SEC has a notice that requires issuers of municipal bonds to disclose "material" events.
The definition of "material" is left to the discretion of the issuer. Issuers have drawn down on bond insurance to meet principal and interest payments when property tax payments have been late.
Technically this "default" has not been considered "material" because the issuers view the draw on bond insurance as a temporary bridge loan. The issuers have repaid the insurers within 90 days but have not disclosed the use of insurance to investors through SEC filings.
The SEC has modified the disclosure language to require material event notices for "all unscheduled draws on credit enhancement facilities reflecting financial difficulties". The new disclosure requirement takes effect in December.
The modification is a distinction without a difference because the determination of "financial difficulties" is a bomb blast away from required disclosure.
Berkshire has written 90% less bond insurance year over year because the capital commitment does not justify the risk, unless it becomes clear that the federal government stands behind municipal debt.