I believe that until about 1995 there were fewer than 5 bond issuers subject to Chapter 11 proceedings (strategic bankruptcies) that were AAA at the time of the Chapter 11 filing: Texaco and, I think, Manville the asbestos maker. There was also one AAA muni bond that defaulted. There were several cases where issuers like Lilco (Long Island Lighting) were rapidly downgraded from AAA to junk.